PURPOSE: To surely reciprocate a piston.

CONSTITUTION: This vibration generating device is constituted of pressure oil of a first pressure receiving chamber 4 of small pressure receiving area, a cylinder part 6 provided with a piston 3 reciprocating by difference of pressure receiving area between the first pressure receiving chamber 4 and the second pressure receiving chamber 5 of large pressure receiving area, a changeover valve 9 provided with a piston 8 alternately allowing the second pressure receiving chamber 5 to communicate with an oil pressure source and a tank by changing to a first position and a second position due to pressure of a first pressure chamber 13 and a second pressure chamber 14, and a servo valve 19 controlling to allow the first pressure chamber 13 of the changeover valve 9 to communicate with the oil pressure source and the tank by movement of the piston 3.