PURPOSE: A flap-type cabin discharge device of a vehicle is provided to supply a pressure relief valve for exhausting the air from a first lateral side of a valve to a second lateral side thereof and cutting off the flow of the air from the second lateral side of the valve to the first lateral side. CONSTITUTION: A pressure relief valve(10) is composed of a housing having a base unit(24) forming an air passage(34) and a flap(90) positioned on the air passage. At least a portion of the flap responds to the air pressure difference acting on the opposite side of the flap and moves for the base unit of the housing. At least a portion of the flap moves between an opening state for exhausting the air from a first lateral side of the valve to a second lateral side and a closing state for cutting off the flow of the air from the second lateral side of the valve to the first lateral side. The flap has plural mounting opening units(104,106). The housing has plural T-shaped mounting posts(70,72) projected from the base unit of the housing. Snap combination is supplied between the flap and the mounting post to fix the flap to the housing. The mounting post is extended through the opening unit. The mounting post is a unique structure for settling the flap on the housing.