A flap valve apparatus includes a main body; a valve body; and a spring member. The main body includes support shafts, and the valve body includes bearing portions. When viewed from the axial direction of the support shaft, the bearing portions has a bearing hole with an inner diameter larger than the outer diameter of the support shaft; and an introduction groove being in communication with the bearing hole from the seal surface side of the valve body. A width of the introduction groove is smaller than the outer diameter of the support shaft, but is expandable to receive the support shaft. When viewed from the axial direction of the support shaft, an opening portion of the introduction grove is positioned near the axial center of the main body.