PURPOSE: To produce an Al-diffused steel sheet having excellent oxidation resistance and high-temp. strength by plating Al contg. a specific ratio of Si on the surface of the specifically composed steel sheet and subjecting the same to a heat treatment under specific conditions.

CONSTITUTION: The Al contg. 3W15% Si is plated on the surface of the steel sheet which contains, by weight %, 0.6W1.5 Mn, 0.03W0.1 P, 0.01W0.08 Al, 0.08W0.25 Ti, and 1 or 2 kinds of ≤0.3 Si and ≤0.003 B, is limited to ≤0.01 C and ≤0.004 N, and consists of the balance substantially Fe. Such steel sheet is subjected to heat-treatment at a temp. and for a time in the range enclosed by the respective points A (5hr, 800°C), B (100hr 800°C), C (100hr, 700°C), and D (10hr, 700°C) in a nitrogen atmosphere. The Al-diffused layer 15W50μ of α-Fe contg. 0.5W10% Al and further the Al, Fe, Si alloy-diffused layer 20W50μ thereon are thereby formed on the surface of the steel sheet, by which the Al-diffused steel sheet having the excellent oxidation resistance and strength at a high temp. of about 700W800°C is obtd.