To provide a power assisted bicycle allowing prevention of a careless drop of a battery case.

This motor-assisted bicycle P1 has a motor 2 for driving a rear wheel 1, plural cell-type rechargeable dry batteries 3 as a power source of the motor 2, the flat box-like battery case 4 housing the batteries 3 in series, and a front carrier 6 for mounting a front basket 5. The front carrier 6 is provided with a bracket 7 for installing the battery case 4. The bracket 7 is provided with a key assembly 9 for detachably installing the battery case 4 to the bracket 7 by operation of a key 8. The inside of the key assembly 9 is provided with a coil spring as an elastic member energized to an oppose direction to the installation operation of the battery case 4, i.e., upward here.