Abstract: A vertical wafer supporting boat is provided in which adequate reaction gas is supplied to the portion of a wafer surface to be treated which is inserted in holding grooves in members of the wafer supporting boat during the heat treatment and thus uniform heat treatment is achieved across the surface of each wafer. The particular groove shape preferably exploits the rotation of the boat in a vertical furnace to thereby uniformly supply the reaction gas on the entire surface to be treated of each of the wafers disposed in the boat. The holding grooves are inclined relative to a direction tangent to a wafer edge in order that the clearance facing the surface to be treated of a wafer is profiled to have an edge-like space by gradually increasing the width in the tangential direction of the wafer and, preferably, the internal groove surface opposed to the surface to be treated of the wafer is inclined to increase in width toward the center of the wafer.