A rotation detection device comprising: a housing portion ; a wheel portion having a rotational axis and an axial front end, the wheel portion being retained by the housing portion such that the wheel portion is rotatable about the rotational axis; a magnet body retained by the housing portion , the magnet body rotating in response to rotation of the wheel portion so that a magnetic field of the magnet body changes; and a magnetic detection portion disposed on the housing portion at a position corresponding to a position of the magnet body , the magnetic detection portion detecting the magnetic field of the magnet body in order to derive a rotational position of a a shaft body (B), wherein the wheel portion allows the shaft body (B) that serves as a detection object to be inserted therein in a direction of the rotational axis from a position axially in front of the wheel portion , and is rotated about the rotational axis when the shaft body (B) is rotated, wherein the wheel portion includes a tubular portion in which the shaft body (B) is insertable, and the tubular portion has a front end corresponding to the front end of the wheel portion , and a projecting edge portion extending away from the rotational axis at the front end of the tubular portion , wherein the housing portion includes a cover portion having a hollow tubular opening portion in which the shaft body (B) is insertable, and wherein the cover portion and the wheel portion are engaged with each other such that the opening portion is located on an inner side of the projecting edge portion ; wherein the wheel portion serves as a rotor. a housing portion ; a wheel portion having a rotational axis and an axial front end, the wheel portion being retained by the housing portion such that the wheel portion is rotatable about the rotational axis; a magnet body retained by the housing portion , the magnet body rotating in response to rotation of the wheel portion so that a magnetic field of the magnet body changes; and a magnetic detection portion disposed on the housing portion at a position corresponding to a position of the magnet body , the magnetic detection portion detecting the magnetic field of the magnet body in order to derive a rotational position of a a shaft body (B), wherein the wheel portion allows the shaft body (B) that serves as a detection object to be inserted therein in a direction of the rotational axis from a position axially in front of the wheel portion , and is rotated about the rotational axis when the shaft body (B) is rotated, wherein the wheel portion includes a tubular portion in which the shaft body (B) is insertable, and the tubular portion has a front end corresponding to the front end of the wheel portion , and a projecting edge portion extending away from the rotational axis at the front end of the tubular portion , wherein the housing portion includes a cover portion having a hollow tubular opening portion in which the shaft body (B) is insertable, and wherein the cover portion and the wheel portion are engaged with each other such that the opening portion is located on an inner side of the projecting edge portion ; wherein the wheel portion serves as a rotor. <EOS>