We present a calculation of the K → ππ decay amplitudes from the K → π matrix elements using leading order relations derived in chiral perturbation theory. Numerical simulations are carried out in quenched QCD with the domain-wall fermion action and the renormalization group improved gluon action. Our results show that the I = 2 amplitude is reasonably consistent with experiment whereas the I = 0 amplitude is sizably smaller. Consequently the ΔI = 1/2 enhancement is only half of the experimental value, and ε′/ε is negative.