To provide a crank arm device preventing rust and damage to a screw part.

A crank arm device includes an axle having a first and a second end part having an axle body and spline, wherein the second end part has an outer peripheral surface and a threaded inner circumference surface, an axle bolt having a threaded outer peripheral surface screwed into the threaded inner peripheral surface of the second end portion of the axle and a crank arm having an axle mounting boss defining an opening for receiving the second end portion of the axle therein. The axle has a flange that abuts on a first crank arm from an axial direction outside and engages with the first crank arm at the first end part and engages with the second crank arm at the second end part. The axle bolt is constructed to engage with the second crank arm at the axial direction outside of the second end part of the axle.