PURPOSE: To measure easily the temp. distribution in a billet during solidification by providing plural pieces of horizontal holes to a metallic rivet having a specific length, packing a metal of the kind different from the billet therein, driving such rivet to the billet under progression of solidification and inspecting said points after solidication.

CONSTITUTION: Plural horizontal holes 11 are provided equidistantly and axially to a metallic rivet 10 having the length of ≥1/2 the thickness of a continuous casting billet 4 and a different metal, for example, Au, FeS, Ni, Cr or the like is preliminarily packed in the holes 11. Such rivets 10 are driven into the billets 4 at the prescribed positions A, B, C of the billet 4 under continuous casting and the crystal condition of the metal in the driven parts after solidification of the billet 4 is investigated by a microscope to measure the temp. distribution during solidification of the billet 4. The temp. distribution in the billet 4 is thus known precisely and the elastic and plastic strain of the billet is exactly known.