To provide a constant differential pressure valve which is compact in size and can be realized at low cost.In the constant differential pressure valve according to the present invention, a pipe forming a body thereof not only accommodates internal structures, such as a differential pressure control mechanism, a movable core, and a fixed core, but also serves as part of piping of a refrigeration cycle. Further, a solenoid section including a solenoid coil is disposed outside the pipe in a manner surrounding the same. Therefore, substantial integration of the differential pressure control mechanism into the piping of the refrigeration cycle can be achieved, which makes the entire constant differential pressure valve very simple in construction. As a result, reduction of the size of the constant differential pressure valve and resultant reduction of material costs and manufacturing costs can be achieved, which makes it possible to achieve low costs of the constant differential pressure valve.