A resolver of a variable reluctance type includes a resolver stator portion of a substantially annular shape centered about a central axis, and a resolver rotor portion which is attached to a shaft of a motor rotor portion arranged inwardly of the resolver stator portion. The resolver is operable to increase accuracy for detecting a rotational angle when a size of gap g between a rotor core and a resolver stator portion at a predetermined point at an outer circumferential edge of the rotor core is set by using a maximum value for the gap g (gmax), a minimum value for the gap g (gmin), angle θ defined by a predetermined point and a reference point, axial double angle n, and the coefficient.