Provided is a magnetic clamp device that is capable of more finely measuring an induced voltage generated in a coil and selecting countermeasures against reductions in magnetization force. Multiple magnet blocks 11, 21 each comprising an invertible magnet 18, the polarity of which can be inverted, and non-invertible magnet 15, are positioned on a surface of a plate PL composed of magnetic body magnetically clamping a mold M1, M2 when in a magnetized state. The magnetic flux traversing the invertible magnet 18; and a control device 33 that determines whether or not there has been a polarity inversion in the induced voltage detected from the coil 31, and if there has been a polarity inversion, warns that the adhesion of the molds M1, M2 has decreased.