To provide a flow channel opening/closing apparatus that can keep the instantaneous flow rate of supply water, quickly open and close a main valve for starting and stopping water supply and be downsized. A flush valve CV, which is such a flow channel opening/closing apparatus, incorporates constant flow rate keeping part that operates to keep constant the main flow rate of water flowing from a primary-side internal flow channel 20 to a secondary-side internal flow channel 30, and the constant flow rate keeping part has a constant flow rate valve body 44 and a constant flow rate valve seat, operates to adjust the distance between the constant flow rate valve body 44 and the constant flow rate valve seat, and is configured as an integral valve member including a main valve body 42 and the constant flow rate valve body 44.