An opening sealing structure is provided with a housing, a step portion, a seal cap, an elastic sealing member, and a cap fixture. The housing has a cap mounting portion having a a cylindrical shape. The step portion is provided on the outer peripheral surface of the cap mounting portion and has a side surface facing the base end side of the cap mounting portion. The side surface of the step portion is inclined along the circumferential direction of the cap mounting portion from the tip end side of the cap mounting portion to the base end side and its inclination angle is smaller on the base end side of the cap mounting portion than on the tip end side. The cap fixture has a cap holding portion movably engaged with the seal cap in a circumferential direction of its outer peripheral surface and an elastic portion connected to the cap holding portion. The elastic portion is provided with a protrusion which engages the side surface of the step portion.