To provide an alloy tool steel having high-temperature strength at an operating temperature of the order of 700.degree. C., while maintaining room-temperature strength as high as that of a conventional matrix high-speed tool steel. The steel contains from 0.45 wt % to 0.65 wt % C, from 0.10 wt % to 1.00 wt % Si, from 0.20 wt % to 2.00 wt % Mn, not more than 0.020 wt % P, not more than 0.015 wt % S, not more than 1.00 wt % Cu, not more than 1.00 wt % Ni, from 3.50 wt % to 5.00 wt % Cr, from 0.00 wt % to 3.00 wt % Mo, from 0.00 wt % to 10.00 wt % W, from 1.00 wt % to 2.00 wt % V, from 0.00 wt % to 8.00 wt % Co, not more than 0.10 wt % Al, not more than 0.01 wt % O, not more than 0.02 wt % N, and the balance substantially constituted of Fe and unavoidable impurities, in which Weq is from 2.0 to 10.0, 2Mo/Weq is not more than 0.60, and .DELTA.C is from -0.3 to 0.0.