To provide an air compressor which can improve reliability by preventing dust from entering a casing of an electric motor.

An air compressor includes: a compressor body 1; an electric motor 2; a gear mechanism 3 that transmits power of the electric motor 2 to the compressor body 1; a gear casing 4 which is connected between a casing for the compressor body 1 and a casing for the electric motor 2 and accommodates the gear mechanism 3; a load side casing 9 and a seal cover 24 that define the internal space of the gear casing 4 and the internal space of an electric motor main casing 8; and a non-contact visco seal 23 provided to a gap in a part where a rotating shaft 5 of the electric motor 2 penetrates through the load side casing 9. In the air compressor, communication holes 26, 27 are formed that communicate between a gap (the space in a channel 25) at the side of the electric motor 2 from the visco seal 23 in the load side casing 9 and the seal cover 24 and the outside of the air compressor not through the internal space of the electric motor main casing 9.