To provide a method of reducing the influence of the surface layer section on the outer surface of steel pipes and rapidly and stably measuring the distribution of depths in carburization along the peripheral direction of the steel pipes.

The method of measuring depth in carburization on the inner surface of steel pipes includes a first step for arranging a sensor 11 formed by winding an excitation coil 112 and a detection coil 113 around a core 111 whose section is in a U shape so that both the end sections 111a, 111b of the core 111 are along the longitudinal direction of the steel pipe P to be measured; a second step for applying voltage at a specific frequency to the excitation coil 11; a third step for extracting harmonics at a specific frequency from a voltage waveform induced in the detection coil 113 by electromagnetic induction generated between the excitation coil 112 and the detection coil 113; and a fourth step for calculating depth in carburization, based on the correlation relationship between the extracted harmonics and the depth in carburization.