Abstract: A coupler which connects a nipple provided on a cylinder valve of a gas supply line in a semiconductor process to an operating fluid supply line, includes: a first inlet port connected to the operating fluid supply line; a housing having a first cavity that is in communication with the first inlet port; at least one ball which is engaged with a ring-shaped engaging recess formed around a connection portion of the nipple; and a pipe portion which is installed in the first cavity and regulates a second cavity in which the connection portion of the nipple is inserted.