A magnetic sensor device includes: a magnetic circuit for forming a magnetic field, a magnetoresistance effect element, and a heat dissipater. The magnetoresistance effect element outputs changes in the magnetic field as changes in a resistance value, and is arranged on a surface (of a +Z side) of the magnetic circuit at a conveyance path side thereof. The heat dissipater is arranged in close contact with the magnetic circuit at the opposite side thereof (−Z side) from the conveyance path. |