PURPOSE: To provide a member for measuring internal temperature and a internal temperature measuring method using the same which enables the determining of setting conditions by measuring a rise in the temperature in a member to be heated under specified heating conditions.

CONSTITUTION: Test piece 10 for surface grinding is so arranged to assemble two rectangular blocks 12 and 14 with almost the same shape and dimensions in a close contact state. The assembled surface between the blocks 12 and 14 in close contact is worked to be smooth relatively. A bismuth thin film 24 is formed n the assembled surface 20b of the one block 14, for example, by vacuum evaporation, sputtering or the like. The test piece 10 is fixed in such a direction as to contact any edge within a ground surface to test and work and after the working, the boundary line of the melting or non-melting area of the bismuth thin film 24 is observed to obtain an internal temperature.