Electrically assisted bicycle

In an inventive electrically assisted bicycle (10), an optimum assist drive force is applied to a front driven wheel (13) according to a traveling environment. The electrically assisted bicycle (10) has a traveling environment load defining table (310) in which plural types of traveling environment loads are predefined by employing a bicycle traveling speed and a torque applied to a crank as variables. A traveling environment load level is detected by fitting a torque detected by a torque detecting unit (43) and a traveling speed detected by a traveling speed detecting unit (42) in the table (310). A control unit (31) sets the ratio (assist ratio) of an assist drive force to be generated by an electric drive system to a drive force generated by a human power drive system based on the load thus detected, and causes the electric drive system to generate the assist drive force at the ratio thus set.