To allow a wide range of existing bicycles to be easily converted into electric power assisted bicycles, a retrofit electric machine for a bicycle includes: a housing (52) configured to be attached to a frame (18) of a bicycle (10), an electric motor (54) attached to the housing (52); an annular rotating plate (84) mounted on the housing (52) in a rotatable manner and connected to the electric motor (54) in a torque transmitting relationship; a pair of rotatable claw members (96) mounted on the rotating member (84) and having parts configured to abut against either rotational side of a crankarm (26) for a pedal of the bicycle (10); and a connecting member (40, 92) configured to connect the rotatable claw members (96) with a rotationally central part of the crankarm (26) such that the rotating plate (84) is concentric to a rotational center line of the crankarm (26).