In an electricity generating device by applying vehicle weight, a rolling vehicle wheel is weighting down on a pressure receiver to actuate a driving mechanism having a restoring function. It is then able to provide a unidirectional rotating torque to drive a generating device to produce electrical power. The mechanical power transmission is performed by a transmission gear unit composed of a driving wheel and a follower wheel interconnected each other with a transmission means therebetween. When the vehicle weight is applied to the driving mechanism by pressing the vehicle wheel on the pressure receiver, the driving mechanism is forced to downwardly urge a ratchet wheel and a driving wheel to rotate that in turn causes the rotation of the follower wheel through the transmission means thereby driving the generating device to operate to produce electricity. The driving mechanism can recover its initial state with the aid of a balancing weight as soon as it has been liberate from the exerted pressure of the vehicle wheel. The device is to be installed flush with the road surface to work.