A magnetic sensor device includes a magnet to form a magnetic field in a conveyance path of a detection target, a magnetoresistance effect element to output a change in the magnetic field as a change in a resistance value, a casing to enclose or hold the magnet and the magnetoresistance effect element, a cover to cover the magnetoresistance effect element and form a conveyance path surface that is a surface along the conveyance path, and a signal amplification board having a signal amplification IC disposed on an intersection surface that intersects the conveyance path surface. The signal amplification IC amplifies the change in the resistance value that is output by the magnetoresistance effect element. The change in the resistance value depends on the change in the magnetic field caused due to conveyance of the detection target on the conveyance path surface.