A variable-camshaft-timing mechanism is provided with a conically spiral spring valve as a check valve. When the conically spiral spring valve is opened, a plurality of flow passage clearances are formed between adjacent windings of the check valve, whereby a pressure loss of the working fluid can be reduced when passing through the check valve. When a reverse flow is generated, the check valve receives the reverse flow in its axial direction. Thus, the reverse flow of the working fluid can be utilized as a thrust force in a close direction of the check valve. A valve closing responsiveness of the check valve can be improved.