A valve opening/closing timing control device, comprising: a driving rotating body that rotates in synchronization with a crankshaft of an internal combustion engine; a driven rotating body that is located on an inner circumference side of the driving rotating body so as to be relatively rotatable, and that rotates in synchronization with a camshaft for opening/closing a valve of the internal combustion engine; a fixed shaft portion by which an inner circumferential part of the driven rotating body is supported so as to be rotatable about a rotational axis that is the same as a rotational axis of the driving rotating body; a fluid pressure chamber that is formed between the driving rotating body and the driven rotating body; an advancing chamber and a retarding chamber that are formed by partitioning the fluid pressure chamber with a partitioning portion that is provided on an outer circumference side of the driven rotating body; an advancing channel that is in communication with the advancing chamber, and a retarding channel that is in communication with the retarding chamber, the advancing channel and the retarding channel being formed in the driven rotating body; and a phase control unit for controlling a rotation phase of the driven rotating body relative to the driving rotating body such that a pressurized fluid is selectively supplied to or discharged from the advancing chamber or the retarding chamber via an inside of the fixed shaft portion and via the advancing channel or the retarding channel, wherein the driven rotating body has: an inner circumferential member that has a cylindrical portion into which the fixed shaft portion is inserted, and a coupling plate portion for coupling the camshaft to one end portion of the cylindrical portion, the cylindrical portion and the coupling plate portion being integrated with each other; and a cylindrical outer circumferential member that is located on an outer circumference side of the inner circumferential member and is provided with the partitioning portion, the outer circumferential member is provided with the inner circumferential member in a unified manner so as to have the same rotational axis, the inner circumferential member is formed with an iron-based material, and the outer circumferential member is formed with a material that is lighter in weight than the iron-based material wherein an opening part of the cylindrical portion extends further in a direction along the rotational axis than a part which is provided with the outer circumferential member in a unified manner. a driving rotating body that rotates in synchronization with a crankshaft of an internal combustion engine; a driven rotating body that is located on an inner circumference side of the driving rotating body so as to be relatively rotatable, and that rotates in synchronization with a camshaft for opening/closing a valve of the internal combustion engine; a fixed shaft portion by which an inner circumferential part of the driven rotating body is supported so as to be rotatable about a rotational axis that is the same as a rotational axis of the driving rotating body; a fluid pressure chamber that is formed between the driving rotating body and the driven rotating body; an advancing chamber and a retarding chamber that are formed by partitioning the fluid pressure chamber with a partitioning portion that is provided on an outer circumference side of the driven rotating body; an advancing channel that is in communication with the advancing chamber, and a retarding channel that is in communication with the retarding chamber, the advancing channel and the retarding channel being formed in the driven rotating body; and a phase control unit for controlling a rotation phase of the driven rotating body relative to the driving rotating body such that a pressurized fluid is selectively supplied to or discharged from the advancing chamber or the retarding chamber via an inside of the fixed shaft portion and via the advancing channel or the retarding channel, wherein the driven rotating body has: an inner circumferential member that has a cylindrical portion into which the fixed shaft portion is inserted, and a coupling plate portion for coupling the camshaft to one end portion of the cylindrical portion, the cylindrical portion and the coupling plate portion being integrated with each other; and a cylindrical outer circumferential member that is located on an outer circumference side of the inner circumferential member and is provided with the partitioning portion, the outer circumferential member is provided with the inner circumferential member in a unified manner so as to have the same rotational axis, the inner circumferential member is formed with an iron-based material, and the outer circumferential member is formed with a material that is lighter in weight than the iron-based material wherein an opening part of the cylindrical portion extends further in a direction along the rotational axis than a part which is provided with the outer circumferential member in a unified manner.