Abstract: A washing apparatus comprises a loader, unloader, and three washing units (i.e., a carry-in side unit, a carry-out side unit, and an intermediate unit) arranged between the loader and unloader. Each unit has two washing sections for performing batch treatment of wafers, and a rotary transfer arm. An underwater loader is provided between each pair of adjacent washing units. Each of the washing sections and underwater loaders has a vessel and a case housing the vessel. The case is provided with a shutter and an air curtain at an opening thereof for isolating the atmosphere therein from the outside thereof. The loader, unloader, washing sections, and underwater loaders are located around the rotational axis of the rotary transfer arm such that each adjacent two are arranged substantially at right angles to each other, which enables the device to be made compact and to have high treatment efficiency.