A position detecting device includes a shaft part that rotates in conjunction with a rotation of an external driving body, a magnet that is integrally provided at one end of the shaft part, a substrate on which a detecting element detecting a magnetic field of the magnet is mounted, a case that includes a disposition portion in which one end of the shaft part and the substrate are disposed, a protective member into which the other end of the shaft part is inserted and which covers the disposition portion, and a seal member that is disposed between the case and the protective member. A support member supporting one end of the shaft part and a bearing supporting the shaft part are mounted on the protective member. The protective member is provided with a protrusion, and the support member is provided with a fitting portion to which the protrusion is fitted.