PROBLEM TO BE SOLVED: To provide a gate-type magnetic body detector that issues a warning of increase of change in magnetic flux density, as means for detecting and reporting entry of a magnetic body into an MRI examination room in order to prevent the magnetic body from being attracted by a strong magnet of an MRI diagnostic apparatus.SOLUTION: In a magnetic body detector 21, a plurality of magnetic sensor sets 1 are installed around a passage 13 of a gate. Each of the magnetic sensor sets 1 is composed of two or three magnetic sensors 2, 3, 4, and respective detection axes 5, 6, 7 of the sensors are arranged at angles of 90±30° including an orthogonal angle to each other. A change in magnetic flux density caused by movements of magnetic bodies 22, 23 is detected by the magnetic sensor sets 1 a scalar and angle &thetas of a vector of the change in magnetic flux density is calculated and if any of the values in the calculation result is greater than a predetermined threshold 17 and if it is detected that the movement is in the direction of entering the room, a warning by sound, light or the like is issued to the user thereby to prevent the magnetic bodies 22, 23 from entering the room.
