The device has a processor (20) that applies a magnetic field to a measurement subject (2) and after the magnetic field is discontinued, uses an induced electromotive force detector (17) to measure the magnetic fluxes emitted from multiple locations, calculates time constants of transient change of the plurality of magnetic fluxes, and detects the internal structure of the measurement subject (2) from the distribution of the time constants. The processor (20) prompts the induced electromotive force detector (17) to make first measurements at predetermined locations, prompts the induced electromotive force detector (17) to make second measurements at locations rotated by a predetermined angle from the predetermined locations, and based on the internal structure detected by the first measurements and the internal structure detected by the second measurements, estimates the center location of the nugget and/or the diameter of the nugget which has been formed inside the measurement subject (2).