Abstract: An actuator is disclosed, which comprises a drive motor, a rotary member to be rotated by the drive motor, and a plunger for reciprocating in its axial direction. The plunger has a first and second guide portions formed thereon. The actuator further comprises a guiding member held in the rotary member. The guiding member includes a first spiral guide surface for guiding the first guide portion therealong to move the plunger in a direction to protrude it, a second spiral guide surface for guiding the second guide portion therealong to move the plunger in a direction to retract it, a first position regulating surface for regulating the retraction of the plunger when it contacts with the first guide portion, and a second position regulating surface for regulating the protrusion of the plunger when it contacts with the second guide portion.