A valve opening and closing timing control apparatus comprising: a driving side rotator disposed rotatably about a rotation axis and configured to rotate synchronously with a crankshaft of an internal combustion engine; a driven side rotator disposed rotatably about the rotation axis and configured to be rotatable relative to the driving side rotator and to rotate integrally with a valve opening/closing camshaft of the internal combustion engine; and a phase adjustment mechanism configured to set a relative rotation phase between the driving side rotator and the driven side rotator by a driving force of an electric actuator, wherein the phase adjustment mechanism includes: an internal toothed ring gear disposed coaxially with the rotation axis; an inner gear disposed coaxially with an eccentric axis in a posture parallel to the rotation axis such that an external toothed portion thereof is engaged with a portion of an internal toothed portion of the ring gear; and a drive shaft fitted into the inner gear, the phase adjustment mechanism is configured with a differential speed reduction mechanism in which the drive shaft is rotated about the rotation axis by the driving force of the electric actuator such that the inner gear revolves around the rotation axis while spinning on the eccentric axis, and the valve opening and closing timing control apparatus further comprises: a biasing member provided on an outer periphery of the drive shaft to apply a biasing force in a direction such that the external toothed portion of the inner gear is engaged with the internal toothed portion of the ring gear; and a displacement regulation portion configured to regulate a displacement of the inner gear on the basis of the drive shaft in a biasing direction of the biasing member such that an engagement depth of the internal toothed portion and the external toothed portion is maintained to a predetermined value, characterized in that the displacement regulation portion is configured by setting an outer diameter of an outer peripheral surface of the drive shaft at a side opposite to the biasing member with the eccentric axis interposed therebetween to the same value as an inner diameter of an inner peripheral surface of the inner gear." a driving side rotator disposed rotatably about a rotation axis and configured to rotate synchronously with a crankshaft of an internal combustion engine; a driven side rotator disposed rotatably about the rotation axis and configured to be rotatable relative to the driving side rotator and to rotate integrally with a valve opening/closing camshaft of the internal combustion engine; and a phase adjustment mechanism configured to set a relative rotation phase between the driving side rotator and the driven side rotator by a driving force of an electric actuator, wherein the phase adjustment mechanism includes: an internal toothed ring gear disposed coaxially with the rotation axis; an inner gear disposed coaxially with an eccentric axis in a posture parallel to the rotation axis such that an external toothed portion thereof is engaged with a portion of an internal toothed portion of the ring gear; and a drive shaft fitted into the inner gear, an internal toothed ring gear disposed coaxially with the rotation axis; an inner gear disposed coaxially with an eccentric axis in a posture parallel to the rotation axis such that an external toothed portion thereof is engaged with a portion of an internal toothed portion of the ring gear; and a drive shaft fitted into the inner gear, the phase adjustment mechanism is configured with a differential speed reduction mechanism in which the drive shaft is rotated about the rotation axis by the driving force of the electric actuator such that the inner gear revolves around the rotation axis while spinning on the eccentric axis, and the valve opening and closing timing control apparatus further comprises: a biasing member provided on an outer periphery of the drive shaft to apply a biasing force in a direction such that the external toothed portion of the inner gear is engaged with the internal toothed portion of the ring gear; and a displacement regulation portion configured to regulate a displacement of the inner gear on the basis of the drive shaft in a biasing direction of the biasing member such that an engagement depth of the internal toothed portion and the external toothed portion is maintained to a predetermined value, a biasing member provided on an outer periphery of the drive shaft to apply a biasing force in a direction such that the external toothed portion of the inner gear is engaged with the internal toothed portion of the ring gear; and a displacement regulation portion configured to regulate a displacement of the inner gear on the basis of the drive shaft in a biasing direction of the biasing member such that an engagement depth of the internal toothed portion and the external toothed portion is maintained to a predetermined value, characterized in that the displacement regulation portion is configured by setting an outer diameter of an outer peripheral surface of the drive shaft at a side opposite to the biasing member with the eccentric axis interposed therebetween to the same value as an inner diameter of an inner peripheral surface of the inner gear."