Rotation of a magnetized rotor is detected in a highly accurate manner without depending on a magnetic pole pitch of N and S poles of the magnetized rotor. To that end, a magnetic detection device includes a magnetoresistive element that is formed of a fixed layer made of a ferromagnetic material whose magnetization direction is fixed and a free layer made of a ferromagnetic material whose magnetization direction can be freely changed, with a non-magnetic middle layer sandwiched between the layers, and arranged therein maintaining a gap between the element and the outer circumferential surface of the magnetized rotor in which the N and S poles are alternately arranged along the outer circumference rotating around a rotation shaft, wherein the magnetoresistive element is arranged in such a way that a plane on which the fixed layer is formed is in a plane including the rotation shaft.