In order to improve its durability, a fluid-pressure actuator is constructed from a swelling/contraction body having opposite ends closed by plug members and swelling and contracting when liquid is supplied to and discharged from the body; a cover body for covering the outer peripheral surface of the expansion/contraction body and having its opposite ends fastened and fixed to the plug members; a fluid supply/discharge opening formed in the plug members; and an auxiliary member for preventing or suppressing local deformation in which the cover body swells outward when the fluid is supplied to the swelling/contraction body.