Abstract: A fluid pressure-type actuator is stably operable for a longer period of time than that of a conventional actuator. The actuator 1 is formed with a non-rubber bag body 5 covered with a covering body 2 that is expandable and contractable. The bag body 5 is constructed so that both a longitudinal dimension and an outer diameter thereof when inflated to the maximum extent are larger than an inner longitudinal dimension and inner diameter of the covering body 2 when the covering body 2 is expanded to the maximum extent. The covering body 2 is constructed so as to have a constricting force against a pressing force caused by inflating the bag body 5 when the covering body 2 is expanded to the maximum extent. If fluid is supplied to the actuator 1, because the bag body 5 is restrained with the covering body 2 before inflated to the maximum extent, an explosion of the bag body 5 is prevented.