PURPOSE: To assist rotation of an engine so as to contrive saving of fuel and reduction of a harmful component in exhaust gas by converting rotational energy of the engine into energy of oil pressure accumulated at the time of deceleration, and releasing this accumulated energy at the time of acceleration.

CONSTITUTION: At the time of deceleration of an automobile mainly by stepping in a foot brake, turning force by a flywheel of an engine 1 is transmitted to the first oil 3 through a gear mechanism 2 thus to accumulate a pressure by press-feeding oil to an accumulator 12 through an oil transport pipe 7 and a branch pipe 11. Here, the second oil pump 9 is driven through a motor 16 by a microcomputer 17, connected to a battery 18, to assist pressure accumulation of oil to the accumulator 12. At the time of acceleration of the automobile, the accumulated oil is released from the accumulator 12, and by functioning the first oil pump 3 as an oil motor by switching a valve 5, the oil is supplied to the engine 1.