A rotation position detection device includes a change drum, a magnetic field generator, and a magnetic field detector. The magnetic field generator is fixed to the change drum and rotates along with the change drum to differentiate an ambient magnetic field. The magnetic field detector is formed separately from the magnetic field generator and fixed to a position at which a magnetic field changes in response to rotation of the magnetic field generator so as to detect the magnetic field and output a signal that depends on the rotational position of the change drum on the basis of the detection result.