We have studied a commensurate to incommensurate transition in the novel helimagnet CuB2O4 by means of high-frequency ESR. In the commensurate weak ferromagnetic (CWF) state, two ESR modes have been found at low temperatures, which are explained in terms of an electron paramagnetic resonance of the Cu2+ spins at 8d-sites and an antiferromagnetic resonance of the magnetically ordered Cu2+ spins at 4b-sites with the Dzyaloshinskii-Moriya (DM) interaction. The DM interaction among the Cu 2+ spins at 4b-sites that carries the CWF state is evaluated quantitatively. The incommensurate helimagnetic state is discussed in terms of the magnetic frustration. ©2008 The Physical Society of Japan.