A bicycle fitted with a bicycle electric power assist device, the bicycle electric power assist device comprising: a housing supported by a frame of a bicycle; an electric motor attached to the housing; a battery serving as a power source for the electric motor; an annular rotational output member rotationally driven by the electric motor, the rotational output member being rotatably mounted on the housing to be positioned around a crank axis line which is a rotational center line of a crankshaft and a crank arm for a pedal of the bicycle; and a connecting member connecting the rotational output member to the crankshaft or the crank arm in a torque transmitting relationship, the housing includes an annular part that concentrically and rotatably supports the rotational output member, wherein the rotational output member and the annular part are positioned between the frame and the crank arm, and the annular part is provided with a tubular portion defining a central opening through which the crankshaft can loosely pass, and the rotational output member is coaxially and rotatably mounted on an outer periphery of the tubular portion, characterized in that the bicycle electric power assist device further comprises a transmission gear train received in the housing to transmit a rotational motion of the electric motor to the rotational output member, the electric motor being displaced radially outward from the rotational output member via the transmission gear train which is positioned between the electric motor and the rotational output member, the rotational output member is provided with an output gear of the transmission gear train and a rotational output plate attached to a side of the output gear, the rotational output plate having a larger outer diameter than the output gear, and the connecting member is a staple-shaped connecting member fixed to the rotational output plate, the connecting member projecting laterally outward from the rotational output member and interposing a base end part of the crank arm from front and rear sides with respect to a rotational direction thereof so as to be in contact with a front surface and a rear surface of the crank arm and slidable along the crank arm in a longitudinal direction of the crank arm. a housing supported by a frame of a bicycle; an electric motor attached to the housing; a battery serving as a power source for the electric motor; an annular rotational output member rotationally driven by the electric motor, the rotational output member being rotatably mounted on the housing to be positioned around a crank axis line which is a rotational center line of a crankshaft and a crank arm for a pedal of the bicycle; and a connecting member connecting the rotational output member to the crankshaft or the crank arm in a torque transmitting relationship, the housing includes an annular part that concentrically and rotatably supports the rotational output member, wherein the rotational output member and the annular part are positioned between the frame and the crank arm, and the annular part is provided with a tubular portion defining a central opening through which the crankshaft can loosely pass, and the rotational output member is coaxially and rotatably mounted on an outer periphery of the tubular portion, characterized in that the bicycle electric power assist device further comprises a transmission gear train received in the housing to transmit a rotational motion of the electric motor to the rotational output member, the electric motor being displaced radially outward from the rotational output member via the transmission gear train which is positioned between the electric motor and the rotational output member, the rotational output member is provided with an output gear of the transmission gear train and a rotational output plate attached to a side of the output gear, the rotational output plate having a larger outer diameter than the output gear, and the connecting member is a staple-shaped connecting member fixed to the rotational output plate, the connecting member projecting laterally outward from the rotational output member and interposing a base end part of the crank arm from front and rear sides with respect to a rotational direction thereof so as to be in contact with a front surface and a rear surface of the crank arm and slidable along the crank arm in a longitudinal direction of the crank arm.