A high speed tool steel, which is high in impact value and free from variations in tool performance, comprising, by mass %, of: 0.4.ltoreq.C.gtoreq.0.9; Si.ltoreq.1.0; Mn.ltoreq.1.0; 4.ltoreq.Cr.gtoreq.6; 1.5 6 in total of either or both of W and Mo in the form of (1/2W+Mo) wherein W.ltoreq.3; 0.5 3 in total of either or both of V and Nb in the form of (V+Nb); wherein carbides dispersed in the matrix of the tool steel have an average grain size of .ltoreq.0.5 .mu.m and a dispersion density of particles of the carbides is of .gtoreq.80.times.10.sup.3 particles/mm.sup.2.