To produce an internal structural material for a zirconium alloy base nulear reactor having sufficient corrosion resistance in a high burnup and long-term use by regulating the total amt. of heat to be inputted to the material after (B quenching to the optimum range and furthermore specifying the componental compsn. of a Zr alloy.

A Zr alloy having a compsn. contg., by weight, 0.8 to 1.6% Sn, 0.19 to 0.45% Fe, 0.04 to 0.10% Ni and Si as impurities having 60 to 200ppm concn., and the balance Zr and satisfying the formula I is melted to form into a slab, which is thereafter subjected to β quenching treatment. After that, rolling and annealing treatments are executed in such a manner that the total amt. ΣA1 of heat to be inputted to the material expressed by the inequality II is regulated to the range of 1×10-19 to 1×10-17. In the inequality, (ti) denotes the treating time (hr) in the heat treating stage on the (i)th after β quenching, T denotes the treating temp. (K) in the stage (l), Q denotes activating energy, R denotes gas constant, and Q/R is regulated to 40000.