data 220 from the table 26 and in accordance therewith outputs the modified pulse to the motor drive section 14 if the 34 set speed 1 34 is selected and the deviation information from the pulse comparator 24 is 60 the modified pulse is calculated to include an on duty period of 6 5 11 ms and an off duty period of 4 5 1 ms in this case the pulse is made to include an on duty period of 10 ms and an off duty period of 0 ms namely if the on duty or off duty period of the calculated pulse is greater than the period of the motor drive pulse or less than 0 zero the larger one is made to coincide with the period of the motor drive pulse and the smaller one is made to coincide with 0 zero the motor drive section 14 outputs the drive voltage to the dc motor 12 based on the modified pulse received from the pulse updating section 28 to change the rotation speed of the dc motor 12 in this manner the dc motor 12 whose rotation speed tends to vary with the load 40 can be desirably controlled in addition when it is necessary to set a new speed and speed information concerning the new speed the operator uses the input device not shown to set on duty period data indicative of the on duty period of the pulse corresponding to the set speed off duty period data indicative of the off duty period cycle period data indicative of the cycle period of the pulse sample period data indicative.