Abstract: A rotation angle sensor has a cylindrical rotating member attached to a cam shaft of an internal combustion engine for rotation therewith, and a pickup. The rotating member has a portion formed on its inner peripheral surface for detection of a crank angle. The pickup has a column-shaped support portion and is attached coaxially with the rotating member with the support portion rotatably inserted into a hole which is formed in a casing of the internal combustion engine. A detecting portion provided at a distal end of the support portion is positioned inside the rotating member to detect a position of the crank angle detection portion during rotation of the rotating member and output a signal. The pickup further has an arc-shaped elongate hole which is formed to extend about a central axis of the support portion, so that the pickup is fixed to the casing of the internal combustion engine with a bolt inserted through the elongate hole.