To realize an actuator, an articulate mechanism device and a robot device capable of attempting to lighten them remarkably in simple constitution.

This actuator is constituted of an expansion means to expand and contract in accordance with inside pressure changes and a direction deciding means to decide an expansion and contraction direction of the expansion means. The expansion means to expand and contract in accordance with the inside pressure changes and a rotation transmission means one end and the other end of which are respectively fixed on first and second constitutional units and to rotate the first constitutional unit against the second constitutional unit in accordance with expansion and contraction of the expansion means are provided on an articulate mechanism device and a robot device.