A valve opening/closing timing control device, including: a drive-side rotating body rotating synchronously with crankshaft; a driven-side rotating body arranged coaxially with the drive-side rotating body and rotating synchronously with a camshaft in an internal combustion engine; a partition section provided to at least one of the drive-side rotating body and the driven-side rotating body so as to partition a fluid pressure chamber into a retard chamber and an advance chamber; a seal member preventing leakage of a working fluid between the retard chamber and the advance chamber; and a biasing member engaging with the seal member by a biasing force caused by elastic deformation and biasing the seal member from the partition section side towards the drive-side rotating body side or the driven-side rotating body side.