Although semiconductor photocatalysis has been investigated actively for a long time, control of dark processes successive to electron transfer from photocatalysts is almost unexplored compared with designing photocatalysts themselves. The present study proposes employment of clay particles as for controlling the dark processes independently of semiconductor photocatalyst particles. We employed niobate-clay binary nanosheet colloids, where colloidal niobate and clay nanosheets are spatially separated at a micrometer level. Niobate nanosheets worked as the semiconductor photocatalyst that released electrons upon UV excitation, and clay nanosheets worked as the turnout switch of the released electrons to determine their destination. When methylviologen (MV2+) molecules that accept the electrons released from niobate were adsorbed on clay nanosheets, reduction of MV2+ predominantly occurred, and hydrogen was little evolved from the colloid. When Pt nanoparticles were deposited on clay nanosheets, photocatalytic hydrogen evolution occurred because Pt loaded on the clay nanoparticles played a role of cocatalyst. When MV2+ and Pt were co-loaded on clay nanosheets, both of MV2+ reduction and hydrogen evolution occurred competitively. The photocatalytic hydrogen evolution carried out by stirring the colloid sample was worse than that conducted without stirring, which indicated positive contribution of the spatial separation of photocatalytic niobate and cocatalytic clay nanosheets.