PROBLEM TO BE SOLVED: To realize a seismograph capable of detecting a vertical or horizontal movement having a long period.
SOLUTION: This detector comprises a means 13 for generating a parallel magnetic field; and a pendulum having a first permanent magnet 14 and a weight 17 connected to the permanent magnet and rotatably arranged in the parallel magnetic field; and a means for detecting the position of the weight 17, wherein the rotation moment by the magnetic force applied to the first permanent magnet 14 and the rotation moment by the gravity applied to the pendulum are formed so as to mutually cancel. The means 13 for generating the magnetic field generates the magnetic field by a second permanent magnet. The first permanent magnet 14 has a vertically long shape, and its center coincides with the rotating center of the pendulum. The weight 17 is connected to one end 18 of the vertically long shape of the first permanent magnet 14.