A magnetostrictive stress sensor (11, 12, 13, 14, 15, 16) includes: a magnetic member {20, (107, 108, 109, 110, 111, 112)} having a magnetostriction; a permanent magnet (30, 35, 113) adjacent to the magnetic member; a magnetic sensor (40, 104A, 104B) for detecting a leak magnetic flux on a side opposite to the permanent magnet with respect to the magnetic member, wherein the leak magnetic flux changes according to a stress acting on the magnetic member and the magnetic sensor detects the change of the leak magnetic flux, to thereby detect the stress acting on the magnetic member, and a direction (21, 108A, 109A) of the stress acting on the magnetic member is substantially orthogonal to a magnetizing direction (31, 31, 113A) of the permanent magnet.