A resolver for a rotation position detection device comprises a rotor portion having a resin gear having a hole formed at a center portion thereof through which a stationary shaft passes, and a rotor core connected to the resin gear; and a stator portion having the stationary shaft, an excitation wiring and a detection wiring, a back core, and a resin housing, wherein the excitation wiring and the detection wiring are arranged around the stationary shaft at a constant distance from the stationary shaft in a diameter direction, the back core is adjacent to or in contact with the stationary shaft, and the resin gear has a shape whose central portion projects in the shaft direction so as to be in contact with the back core.