To provide a deceleration energy regenerating device having improved energy recovery efficiency at the time of deceleration.

A pump-cum-motor 210 pressurizes oil in an oil tank when a vehicle can regenerate deceleration energy and sends the oil to an oil room 222 of an accumulator 220 to move a piston 221, whereby gas filled in a gas room 223 is compressed to recover and store deceleration energy. At the same time, a return control valve 107 is opened to introduce exhaust into a suction pipe in the downstream side of a throttle valve 102. When a vehicle is accelerated, the oil pressurized and sent by the pressure of the pressurized air in the gas room 223 of the accumulator 220 is pressurized and sent to the pump-cum- motor 210 to rotate it.