An endoscopic fluid supply conduit system for use with an endoscope having an insertion tube and a universal cable connected to and from a manipulating head assembly. A fluid conduit is provided internally of the insertion tube to supply a fluid from a fluid feed port on the manipulating head assembly toward a fluid jet injection port feed port which is provided on a rigid tip end section at the fore distal end of the insertion tube. In addition, a second fluid conduit is provided internally of the universal cable and toward the manipulating head assembly. A fluid supply channel selector is provided at the fluid feed port to which a fluid feed adaptor is to be disconnectibly connected. The fluid supply channel selector is adapted to block a fluid flow toward the second fluid conduit when a fluid is supplied to the first fluid conduit from the fluid feed port on the manipulating head assembly, and to bring the second fluid conduit into communication with the first fluid conduit when the fluid feed port on the manipulating head assembly is closed.