The valve opening/closing timing control device includes: the driving rotating body; the driven rotating body; an advancing chamber and a retarding chamber formed by partitioning a fluid pressure chamber between the driving and driven rotating bodies; and a phase control unit supplying pressurized fluid to the advancing or retarding chamber via an advancing channel or a retarding channel penetrating through the driven rotating body. In the driven rotating body, an outer circumferential member and an inner circumferential member are formed integrally/coaxially with each other. The advancing and retarding channel form a predetermined angle. Between every pair of an advancing channel and a retarding channel, a groove portion is formed in one of an inner circumferential surface of the outer circumferential member and an outer circumferential surface of the inner circumferential member, and an elongated protruding portion is formed on the other, at a position that corresponds to the groove portion.