To provide a drive for a power-assisted bicycle, which enhances heat radiation from a driving motor built in a rotary drum of a wheel section and quickly drains infiltrating water.

In assisted operation, when the driving motor bears large load and temperature in the rotary drum rises, heat is properly radiated and temperature rise in the DC brush motor can be prevented. This is because that two brushes 14 are arranged in a lower portion at an acute angle with respect to each other and a plurality of air vents 23 for motor cooling are formed in a side of the rotary drum 8. When rainwater infiltrates into the rotary drum 8 through the air vents 23 for motor cooling and through a plurality of air vents 24 serving also for water drainage, the rainwater is discharged outside through the air vents 24 that serve also for water drainage and face down. The rainwater is prevented from causing high humidity in the rotary drum and from damaging parts of a control circuit or the like and does not cause problems of layer short and the like due to rust and insulation decrease.