An apparatus for measuring a carburized portion occurring in inside of a tubular article, the apparatus comprising a first detecting unit having opposite magnetic poles of a first magnet arranged in parallel to an inspecting face of a case made of nonmagnetic material and a magnetism detector disposed at an intermediate portion between the poles, a second detecting unit having opposite magnetic poles of a second magnet arranged perpendicular to the inspecting face of the case and a magnetism detector provided for one of the poles of the second unit, and a data processing circuit for processing the output signals from the first and second detecting units to produce a difference signal. An influence on the magnetic flux density due to the presence of the carburized portion occurring in inside of the tubular article can be determined by a method comprising the steps of measuring a variation in the magnetic flux density effected from both a deteriorated portion and a carburized portion, each of the portions having high permeability of magnetism, with use of the first detecting unit, measuring a variation in the density of magnetic flux due to the presence of the deteriorated layer by using the second detecting unit, and producing a signal representing the difference between the outputs of the first and second detecting units.