PURPOSE: To provide a motor-operated device for a bicycle which is simply mounted on an ordinary bicycle to form a motor-operated type and suitable for reduction of size.

CONSTITUTION: In a motor-operated drive device for a bicycle mounted on a bicycle and driven by an electric motor 62 in combination with drive by manual drive. Support members 68a and 68b removably mounted on a car body are provided with a pair of right and left drive rollers 60b and 60a brought into rolling contact with the side of the rim 38B of a wheel and nipping the rim 38B from both sides; and an electric motor disposed in the vicinity of the outer periphery of a wheel and rotationally driving the two drive rollers. It is desirable that a battery and a controller are mounted directly on a car body differently from support members 68a and 68b. The motor-operated drive device is removably mounted on the car body of a bicycle and it is desirable that the two drive rollers 60a and 60b are pressed against or separated away from the rim 38B or a tire 38A by a manual lever 106.