An apparatus is provided to measure a depth of a hardened layer formed at a surface layer of a quenched workpiece. The apparatus includes an exciting coil configured to generate a magnetic flux to magnetize the workpiece and a detecting coil configured to detect the magnetic flux generated by the exciting coil. The exciting coil has a U-shaped excitation core portion and an excitation coil portion wound on the excitation core portion. The excitation core portion is arranged such that distal ends of magnetic poles of the excitation core portion face the workpiece. The detecting coil has a detection core portion and a detection coil wound on the detection core portion. The detection core portion is arranged between the magnetic poles of the excitation core portion and along a surface of the workpiece.