PURPOSE: To reduce the size and cost of an air booster by forming a lock valve as a changeover valve wherein pressure set by means of a pressure setting means and pressure in a feedback signal passage are made to be pilot pressures opposing each other.

CONSTITUTION: A lock valve 30 is formed as a changeover valve which supplies air pressure to pilot control means 20, 21 when the air pressure from a discharge port 16 is higher than a set pressure, and discharges the air pressure of the pilot means 20, 21, when it is lower than the set pressure, wherein pressure set by means of a pressure setting means and air pressure from the discharge port 16 are made to be changeover/control pressures. Further, air supply in two spaces for boosting and operation of a cylinder 1 is changed over by switching a main changeover valve 19 through the pilot control means 20, 21. It is possible to reduce the size and cost of an air booster with this arrangement.