A magnetic sensor device that includes: a bar-shaped magnet; a soft magnetic carrier that is arranged parallel to magnet along the longitudinal direction of magnet, that has a magnetoresistive effect element on a surface thereof opposite to a surface facing magnet, and that extends across the lateral length of magnet; and a guide that has a bottom interposed between magnet and carrier and a side wall standing upright from the bottom along a side of magnet contacting a surface of magnet facing carrier, the bottom and the side wall being formed of a nonmagnetic body contacting magnet and extending in the longitudinal direction of magnet. The magnet is attracted to and held by the carrier, with the guide interposed therebetween, due to the magnetic attractive force between the magnet and the carrier.