A high speed tool steel, which is high in impact value and free from variations in tool performance, comprising, by mass %, of: 0.4≦C≧0.9; Si≦1.0; Mn≦1.0; 4≦Cr≧6; 1.6-6 in total of either or both of W and Mo in the form of (½W+Mo) wherein W≦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 ≦0.5 μm and a dispersion density of particles of the carbides is of ≧80×103 particles/mm2.