Five others were injured in Monday evening's blast at the Eagle Fireworks factory in Sangli district.
It was not immediately clear how the fire started.
Accidental explosions are common at Indian fireworks factories, which provide fireworks for weddings, festivals and other ceremonies.
In 2012, 34 people were killed and 30 others injured in a massive blaze at a fireworks factory in the southern Tamil Nadu state.
The BBC Hindi's Devidas Deshpande in Pune says that the latest explosion in Kavathe Ekand village was so powerful that it was heard at least five kilometres away, and the debris of the factory was scattered far and wide.
The mother and the son of the factory owner were among the dead, while his brother and three workers were seriously injured.
The village has many households making firecrackers for local religious festivals and such incidents have been reported before.
Some 25 people have been killed in at least six such incidents in the village in the past few years, our correspondent says.

F-35 air operations were paused at an Arizona air base after pilots suffered symptoms resembling hypoxia, or oxygen deprivation, Captain Mark Graff said.
Lockheed Martin, which built the jet, said it still plans to show the plane at the Paris Air Show this month.
Air Force officials said F-35 fighter jets at other bases were still in use.
Luke Air Force Base, located northwest of Phoenix, cancelled local flying operations for its F-35A Lightning II aircraft on Friday after five incidents were reported in which pilots experienced hypoxia-like symptoms.
But they continued to be grounded on Monday as the Air Force investigated the issue with pilots, maintenance workers and medical professionals, according to base spokeswoman Major Rebecca Heyse.
The incidents took place between 2 May and Thursday, but in each case the jet's backup oxygen system was successful and the aircraft landed safely.
Luke Air Force Base has 55 F-35A Lightning II jets in operation and conducts 25 training flights each weekday, according to base officials.
More than 220 operational F-35 jets have been built worldwide. The stealthy jets have logged more than 95,000 flight hours, but the planes have yet to see combat.

The Post Office said the changes would help to keep branches where customers wanted and needed them to be.
But the Communication Workers' Union (CWU) said the move was "evidence that the Post Office is in crisis".
The union added that the number of post office jobs lost since the beginning of last year was 2,000.
The CWU held a five-day strike before Christmas at Crown post offices in a dispute partly about branch closures. The union is also concerned about job losses and pension provision.
Crown post offices are the larger branches usually found on High Streets.
In a statement, the Post Office said: "With consumer habits changing, and the high cost of maintaining premises in prime high street locations, franchising helps us to keep services where our customers want and need them."
It said it would take time to identify the right partners over the coming months and all proposals would be subject to local consultation. It promised to keep staff "fully informed".
CWU general secretary Dave Ward said: "The latest round of closures is further evidence that the Post Office is in crisis and that the board of the company, backed by the government, is simply pursuing a strategy of slash and burn.
"Today's announcement comes less than three weeks after the closure of a major government consultation on the future of the Post Office and sticks two fingers up to everyone who took part in this."
The union said that 75,000 postcards had been sent by members of the public to the government calling for an end to the closure and franchise programme of the Post Office.
It added that the Post Office network has been reduced by more than 50% over the past 30 years.
In April 2016, the Post Office announced plans to transfer up to 61 branches into WH Smith stores over the following year.
It said the move was part of a 10-year plan to cut costs and save cash, and would act as a way of "safeguarding the future of the network".
At least nine people have been killed in an explosion at a fireworks factory in the western Indian state of Maharashtra, police say.
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The US Air Force has grounded a fleet of F-35 fighter jets indefinitely over concerns over pilots' oxygen supplies, according to a spokesman.
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The Post Office is planning to close and franchise 37 Crown offices, with, the union says, the loss of 300 jobs and 127 specialist roles.