There is provided a rotation angle sensor which does not require resin and an O ring for sealing and is applicable to such a structure that a rotating body to be detected passes through a center portion. A waterproof surface on the main body side 39 positioned at the inner periphery of the circuit-board arrangement portion 27 and rising toward the cover 17 is circumferentially provided at the inner periphery of the housing main body 15, the waterproof surface on the main body side 39 opposes the outer periphery of the detection shank 3 through a space across from the upper to the lower portion of the housing main body 15, a waterproof surface on the cover side 43 disposed opposite to the upper inner periphery of the waterproof surface on the main body side 39 is circumferentially provided at the lower surface of the cover 17 and a ditch D passing between the upper and the lower portion of the housing 5 is formed of the space between the detection shank 3 and the housing 5 and between the detection shank 3 and the waterproof surface on the main body side 39.