In an expansion actuator employing a screw feed mechanism, when a screw feed mechanism (60) formed from a male thread member (58) and a female thread member (59) that are screwed one into the other is driven by a motor (33) via a reduction gear (44), relative displacement between the male thread member (58) and the female thread member (59) in the direction of an axis (L) can be outputted as expansion and contraction due to relative movement of first and second housings (31, 32). Since the screw feed mechanism (60) is disposed so as to surround outer peripheries of the motor (33) and the reduction gear (44), which are disposed on the axis (L), compared with a case in which the motor (33), the reduction gear (44), and the screw feed mechanism (60) are disposed in series on the axis (L), it is possible to make the dimension of the expansion actuator (14) in the axis (L) direction smaller.