Disclosed is a rotation angle detecting device wherein a filling member applied in a second housing section which houses an electromagnetic conversion element is prevented from entering a first housing section which houses a magnet. The rotation angle detecting device (1) is provided with: a rotating member (2); the magnet (3), which is fixed to the rotating member (2) and rotates with the rotation of the rotating member (2); a magnetism detecting package (4) that houses an electromagnetic conversion element (4a), which is disposed to face the magnet (3), and detects the intensity of the magnetic field of the magnet (3) by means of rotation of the magnet (3); a housing (5) having a first housing section (5b) that rotatably houses the rotating member (2), and a second housing section (5d) that houses the magnetism detecting package (4), with a partition (5f) as a reference; a filling member (10), with which the second housing section (5d) is filled such that the magnetism detection package (4) is hermetically housed and held; and a cylindrical metal member (11), which is provided in the first housing section (5b) such that the rotating member (2) is rotatably supported and a part (11a) of the metal member is exposed to the second housing section (5d).