a micro bubble generator the micro bubble generator 25 feeds ballast water mixed with air and ozone and generates micro bubbles the micro bubble generating mechanism being observed focusing only on the air out of the air and ozone fed to the ballast water oh u2212 on the interface with bubbles increases and charges the interface negatively this oh u2212 is free radical species of active oxygen and has oxidative and microbicidal functions in this way microorganisms in the ballast water are killed oh u2212 is generated in large quantity when the micro bubbles crush that is in this embodiment generated micro bubbles show sterilization effect through interaction between electrifiability of micro bubbles and crush of micro bubbles this allows reduction of the quantity of ozone to be mixed the quantity of ozone to be mixed can be within the range of 1 20 ppm for the ballast water the micro bubble generator 25 is preferably represented by a static mixer which does not require power source the pressure loss of the micro bubble generator 25 is preferably within the range of 0 2 0 3 mpa 2 3 kgf cm 2 fig7 shows another example of devices which reduce the quantity of ozone to be mixed in the ballast water 100 in fig7 numeral 26 is a jet generating device which is installed on the pipe 3 between the static mixer 4 and the pressure tank 1 the jet generating device 26 consists of a water jet nozzle 27 and an impact plate 28.