of clean room 1 is a filter 5 which can capture fine grains having a size of 0 03 u03bcm or more external air is fed through filter 5 into room 1 since filter 5 is provided covering the entire region of the ceiling a flow of air from the ceiling toward the floor is formed in the clean room an ion generator 7 is also provided on the ceiling of room 1 it is called a pulsed dc system and a pair of positive and negative special tungsten needle electrodes 8 is provided at an interval of 29 cm the pair of electrodes 8 are impressed with u00b1 13 u00b1 20 kv dc voltages at intervals of 1 11 seconds to thereby generate positive and negative ions alternately from the positive and negative needle electrodes 8 respectively a draft pipe 13 is provided in clean room 1 to purify the internal air in the clean room and to feed it to the vicinity of the needle electrodes 8 room 1 has an air discharge port 14 through which the air in the room is fed to draft pipe 13 feed ports 15 for the purified air are provided in the vicinity of corresponding needle electrodes 8 draft pipe 13 connects air discharge port 14 and air feed ports 15 draft pipe 13 has an air pump 11 the blowing force of which draws the air in room 1 through air discharge port 14 and discharges it from feed ports 15 an.