In an actuator, displacement resistance in axial movement of a slider is reduced to improve durability and enhance maintainability. At each side of the actuator, a slit is made between a top cover and a side cover. Magnetic bodies are provided on the surfaces of both side walls of the slit which face a fixing block of a slider and a magnetic fluid with a viscosity, surface tension, etc. is held between the magnetic bodies. At each side, the fixing block of the slider axially moves along the slit which is closed by the magnetic bodies.