A spring-return, pneumatic actuator includes a casing having therein a cylinder chamber, a housing chamber and a spring chamber and equipped therein with a piston reciprocating by air supplied and discharged from an air supply and exhaust vent on a primary side of the pistons. A piston rod is joined to the piston, and a conversion mechanism is provided for converting a reciprocating motion of the piston into a rotary motion. A spring undergoes a snap action due to the reciprocating motion of the piston, and a communication space chamber is formed via a communication part to have a predetermined volume from a secondary side to the spring chamber. The cylinder chamber has the piston and the piston rod built-in, the housing chamber has the conversion mechanism built-in and the spring chamber has the spring built-in.