To provide an assist power device for a bicycle which can be easily constituted by reducing a cost and mounted after in an existing bicycle.

This device comprises a drive unit 3 and a battery unit 5 supplying power to the drive unit 3. The drive unit 3, in a frame 310 journalled turnably to an axle shaft 6 of a rear frame 2 in a side of a bicycle main unit 1, is provided with an assist sprocket 314 meshed with an intermediate position of a chain 9 driven by a driving motor 312 to be endlessly suspended between front/rear sprockets 7, 8 of the bicycle main unit 1. Further, a spring 316 linked between the rear frame 2 and the frame 310 to make the frame 310 always tilt front and a switch 319 for driving the motor switched off by a front tilt switched on by a rear tilt of the frame 310 are provided.