We propose a codoping method using acceptors and donors simultaneously in order to solve the crucial doping problem of wide-band-gap semiconductor, ZnO. It is very difficult to obtain low-resistivity p-type ZnO while it is very easy to fabricate n-type highly doped ZnO with good conductance. The deliberate codoping of donors with acceptors is essential for the enhancement of acceptor incorporation with a decrease in the lattice energy and a decrease in the binding energy of the acceptor impurity in p-type highly doped ZnO. The donor is not the p-type killer but a good by-player that activates acceptors, i.e. the reactive codopant. The confirmation of the applicability of its codoping method using N acceptor and Ga species as reactive codopant to produce low-resistivity p-type ZnO was achieved experimentally.