PURPOSE: To reduced the electric conductivity to the range where the magnetic field does not diminish, to improve the proof stress at normal temp. and the heat resistance of the title material and to facilitate its atmospheric melting by specifying the compsn. of the title mold material.

CONSTITUTION: The mold material contains, by weight, 0.4W4.0% Ni, 0.1W1.0% Si, 0.05W5.0% Zn, 0.1W2.0% Sn, 0.02W1.0% Mn, 0.001W0.01 % Mg and one or more kinds among 0.001W0.01% Cr, Ti and Zr and the balance consisting of Cu. In the material, since Ni and Si form the deposits of Ni and Si contributing to the improvement of the strength, the above range must be independently incorporated thereto; and since Ni regulates its electric conductivity to the required value, ≤2.0% is needed, and because of the improvement of its strength, ≥0.1% is needed. The molten metal is oxidized to reduce its fluidity at the time of >0.01 % Mg. Cr, Ti and Zr improve the fining of the crystal grains of the ingot and its expansibility at high temp., but in the case of >0.01% additioning thereof, the oxidizability of the molten metal is made severe.