The number n of N-poles and S-poles circumferentially aligned on the cylindrical magnet that integrally rotates with the first shaft is determined so as to be a greatest integer that satisfies the following formula that includes a maximum value Tmax (Nm) of the torque to be detected and a torsion spring constant k (Nm/deg) of the torsion bar connecting the first shaft and the second shaft: 2·n·Tmax/k