A mounting structure for a water blocking cap is adapted to a wiper device that includes a shaft holder and a water blocking cap. The shaft holder includes a stem and a fixed portion. The water blocking cap includes a mounting portion and a water receiver. One of the shaft holder and the water blocking cap includes a guide portion that moves the water blocking cap in a direction intersecting with the axial direction of the stem. In a mounted state in which the mounting portion is mounted to the stem, a part of the water receiver is arranged in a position that is closer to the proximal side of the stem than the fixed portion and in which the part overlaps with the fixed portion as viewed from the axial direction of the stem.