A wheel-supporting roller-bearing unit with air compressor that is compact and capable of keeping resistance against the rotation of the hub 4c low while at the same time maintaining good balance, and when necessary, is capable of obtaining compressed air having sufficient pressure regardless of the traveling speed of the vehicle. A plurality of cylinder holes 53 are located at a plurality of locations that are evenly spaced around the circumferential direction on the inside of a cylinder block 50 that is connected and fastened to the inside end section in the axial direction of the hub 4c. Pistons 54 fit inside these cylinder holes 53 and are moved back and forth by a piston-drive mechanism 62 as the cylinder block 50 rotates. The air compressor 2c is a reciprocating-piston compressor, so it is possible to obtain the required air pressure regardless of rpm. Also, by placing a plurality of cylinder holes 53 and pistons 54 at uniform intervals around in the circumferential direction, it is possible to suppress the resistance against the rotation of the hub 4c.