A cast aluminum alloy excellent in the relaxation resistance property, comprising 9 to 17% by mass of Si, 3 to 6% by mass of Cu, 0.2 to 1.2% by mass of Mg, 0.2 to 1.5% by mass of Fe, 0.1 to 1% by mass of Mn, a balance consists of Al and unavoidable impurities, wherein a Ni content is not more than 0.5% by mass. The average hardness is adjusted to HV130 to HV160 by performing, after casting, solution heating by retaining the alloy at a treatment temperature of 450 to 510° C. for 0.5 hour or longer, performing water quenching and, thereafter, performing aging treatment by retaining the alloy at a treatment temperature of 170 to 230° C. for 1 to 24 hours.