A Co-based alloy containing not less than 0.001 mass % and less than 0.100 mass % of C, not less than 9.0 mass % and less than 20.0 mass % of Cr, not less than 2.0 mass % and less than 5.0 mass % of Al, not less than 13.0 mass % and less than 20.0 mass % of W, and not less than 39.0 mass % and less than 55.0 mass % of Ni, with the remainder being made up by Co and unavoidable impurities, wherein the contents of Mo, Nb, Ti and Ta which are included in the unavoidable impurities are as follows: Mo less than 0.010 mass %, Nb less than 0.010 mass %, Ti less than 0.010 mass %, and Ta<0.010 mass %.