A low alloy steel, characterized by consisting of, by mass %, C: 0.2 0.55%, Si: 0.05 0.5%, Mn: 0.1 1%, S: 0.0005 0.01%, O(Oxygen): 0.0010 0.01%, Al: 0.005 0.05%, Ca: 0.0003 0.007%, Ti: 0.005 0.05%, Cr: 0.1 1.5%, Mo: 0.1 1% and Nb: 0.005 0.1%, and the balance Fe and impurities; and also characterized by the impurities whose contents are restricted to P.ltoreq.0.03% and N.ltoreq.0.015%; and further characterized by containing composites of inclusions of not greater than 7 .mu.m in major axis with appearance frequency of not less than 10 pieces of composites per 0.1 mm.sup.2 of the steel cross section, wherein the composite comprises an outer shell of carbonitride of Ti and/or Nb surrounding a nucleus of oxysulfide of Al and Ca.The low alloy steel suppresses pitting caused by inclusions and suppresses SSC induced by pitting.