PURPOSE: To provide a speed control valve which can be used for a long term by providing a cap-form supporting member which is provided so as to be externally inserted into a cylinder member and receives the large diameter part of an elastic diaphragm and the peripheral surface part excluding the vicinity thereof without interruption in the whole circumference.

CONSTITUTION: This speed control valve is provided with a cylinder member 3 which is fixedly disposed on a coaxial line with a needle valve 2 inside a joint main body 1, an elastic diaphragm 4 externally inserted into the cylinder member 3 and a supporting member 5 which is externally inserted into the cylinder member 3 in order to receive the elastic diaphragm 4. The elastic diaphragm is a cone made of synthetic rubber and is constituted of a relatively thick bottom wall part 40 and a relatively thin conical piece part 41. In the speed control valve, the whole circumference of the conical piece part 4 is deformed so as to be uniformly expanded and contracted, and the elastic diaphragm 4 is not damaged in a short term, so that the speed control valve may be used for a long term.