There is provided a microdevice in which a plurality of fluids L1, L2 and L3 are caused to pass through respective fluid supply passages and flow together in one mixing reaction flow path, and the fluids are diffused in the normal direction of the contact interface thereof while being allowed to flow as a thin layer shaped laminar flow, by which mixing or reaction is carried out. The fluid supply passages are of a concentric multiple cylindrical construction to concentrically laminate the fluids L1, L2 and L3 joined together in the mixing reaction flow path, and the opening width near a joining point of the fluids is reduced to contract the flow.