An underwater actuator (11, 13) comprising: a housing to be immersed under water; a cylinder chamber formed in the housing; a piston accommodated in the cylinder chamber such that the piston is movable in a sliding manner in the cylinder chamber, the piston dividing the cylinder chamber into a first pressure receiving chamber and a second pressure receiving chamber; a rod extending from the piston to the first pressure receiving chamber side, the rod penetrating the housing; a release chamber formed in the housing, the release chamber having an internal pressure kept lower than a water pressure of outside of the housing; and a switching mechanism including: a first switcher configured to switch a communication state between the second pressure receiving chamber and the outside of the housing to allow or block communication therebetween; and a second switcher configured to switch a communication state between the second pressure receiving chamber and the release chamber to allow or block communication therebetween, characterized in that a compressible fluid is encapsulated in the first pressure receiving chamber, the cylinder chamber includes: a movement region, in which the piston moves between a rod-expanded position and a rod-retreated position, the movement region forming the second pressure receiving chamber and a part of the first pressure receiving chamber; and an auxiliary region, into which the compressible fluid of the movement region flows when the piston moves from the rod-retreated position to the rod-expended position, the auxiliary region forming a remaining part of the first pressure receiving chamber, and a pressure in the auxiliary region is lower than the water pressure of the outside of the housing when the piston moves from the rod-retreated position to the rod-expanded position. a housing to be immersed under water; a cylinder chamber formed in the housing; a piston accommodated in the cylinder chamber such that the piston is movable in a sliding manner in the cylinder chamber, the piston dividing the cylinder chamber into a first pressure receiving chamber and a second pressure receiving chamber; a rod extending from the piston to the first pressure receiving chamber side, the rod penetrating the housing; a release chamber formed in the housing, the release chamber having an internal pressure kept lower than a water pressure of outside of the housing; and a switching mechanism including: a first switcher configured to switch a communication state between the second pressure receiving chamber and the outside of the housing to allow or block communication therebetween; and a second switcher configured to switch a communication state between the second pressure receiving chamber and the release chamber to allow or block communication therebetween, characterized in that a compressible fluid is encapsulated in the first pressure receiving chamber, the cylinder chamber includes: a movement region, in which the piston moves between a rod-expanded position and a rod-retreated position, the movement region forming the second pressure receiving chamber and a part of the first pressure receiving chamber; and an auxiliary region, into which the compressible fluid of the movement region flows when the piston moves from the rod-retreated position to the rod-expended position, the auxiliary region forming a remaining part of the first pressure receiving chamber, and a pressure in the auxiliary region is lower than the water pressure of the outside of the housing when the piston moves from the rod-retreated position to the rod-expanded position. a first switcher configured to switch a communication state between the second pressure receiving chamber and the outside of the housing to allow or block communication therebetween; and a second switcher configured to switch a communication state between the second pressure receiving chamber and the release chamber to allow or block communication therebetween, characterized in that a compressible fluid is encapsulated in the first pressure receiving chamber, the cylinder chamber includes: a movement region, in which the piston moves between a rod-expanded position and a rod-retreated position, the movement region forming the second pressure receiving chamber and a part of the first pressure receiving chamber; and an auxiliary region, into which the compressible fluid of the movement region flows when the piston moves from the rod-retreated position to the rod-expended position, the auxiliary region forming a remaining part of the first pressure receiving chamber, and a pressure in the auxiliary region is lower than the water pressure of the outside of the housing when the piston moves from the rod-retreated position to the rod-expanded position. a movement region, in which the piston moves between a rod-expanded position and a rod-retreated position, the movement region forming the second pressure receiving chamber and a part of the first pressure receiving chamber; and an auxiliary region, into which the compressible fluid of the movement region flows when the piston moves from the rod-retreated position to the rod-expended position, the auxiliary region forming a remaining part of the first pressure receiving chamber, and a pressure in the auxiliary region is lower than the water pressure of the outside of the housing when the piston moves from the rod-retreated position to the rod-expanded position.