A provided electric assisted bicycle is configured such that an internal derailleur is provided in a drive unit that internally combines a human driving force and an auxiliary driving force, and the electric assisted bicycle can automatically change gears. The drive unit disposed at an intermediate position between a front wheel and a rear wheel includes a transmission mechanism (25) that transmits a resultant force of a human driving force and an auxiliary driving force from a motor while changing multiple gears. The transmission mechanism (25) includes a transmission weight (81) whose orientation is changed according to a centrifugal force during a rotation of the transmission weight (81). An increase in the rotation speed of a crank shaft (7a) changes the orientations of the transmission weights (81) so as to change from a low gear to a high gear.