PURPOSE: To provide a changeover mechanism for an ejector with simplified operation and with a reduced protrusion part protruding to the outside of a gun.

CONSTITUTION: An ejector mechanism is operated by bringing a protruder 12a of a hammer into contact with a head 13a of a trip rod 13 owing to the hammer to be rotated and returned when a gun is folded after explosion thereof, and hereby moving the trip rod 13 longitudinally. A spiral guide engaging with the trip rod 13 is formed in a selector button 20 located in the vicinity of the trip end 13 and a groove 20b for rotation operation is formed in an end surface of the button. Once the selector button 20 is moved in a direction intersecting the length thereof and hence the protruder 12a is changed over to a location where it makes contact with the head 13a or to a location where it does not make contact with the same.