To form an unleaky type and simultaneously enable temperature compensation.

Relaitng to an electric solid actuator provided with a hydraulic transmission ratio changing device for operating particularly a valve in order to operate a control means. This actuator is provided with a pre-load chamber 4 charged with liquid, one side of this pre-load chamber 4 is partitioned by a diaphragm 3 connected to an actuator main unit 2, the other side of the pre-load chamber 4 is partitioned by an elastic plate main unit 5 of large area. The plate main unit 5 partitions one side of a transmitting chamber 7, the other side of the transmitting camber is partitioned by an elastically supported piston main unit 8 of small area. This piston main unit 8 acts in the control means 14, a valve opening 21 is provided in the plate main unit 5, this valve opening 21 connects the pre-load chamber 4 and the transmitting camber 7, and the opening can be closed by a valve unit 22 connected to the diaphragm 3.