In a previous WISH experiment, we have succeeded in determining the incommensurate cycloidal magnetic structure of AgFeO2, which induces a ferroelectric polarization[1]. However, the correlation length of the cycloidal ordering was not infinite (~ 50 nm) even at the lowest temperature. The macroscopic domain is considered to be spin helicity domain (right-handed or left-handed) in the cycloidal structure, which corresponds to the ferroelectric domain. In parallel, we also investigated how the magnetic and dielectric ground state are affected by partial substitution on the A-site in AgxCu1-xFeO2. We would like to investigate whether the macroscopic domain size can be controlled by an electric field aligning the ferroelectric domains in multiferroic AgxCu1-xFeO2, by performing neutron diffraction experiment under electric fields on WISH.