An improvement to the melt transesterification process for the preparation of polycarbonate is disclosed. The second face of the porous metal sheet defines a flow field while that portion of the porous metal sheet filled with the gas diffusion layer forms a current collector. The horizontal synchronization unit is on a position of an upstream side of scanning lines forming the latent image formed by the plural light beams and deflects that the light beam forming the latent image formed by the plural light beams reaches a predetermined position.