A rotor of an eddy current type retarder produces a brake force by producing eddy current using magnets (4; 34) mounted on a stationary member of the retarder. The rotor is mounted on a rotatable shaft (2; 32) and faces the magnets. The base member (11; 21; 36) is made from a mateiral containing steel. A thin film (13; 15) is formed on that face of the base member (11; 21; 36) which faces the magnets (4; 34). The film is made from a material containing rust prevention material. A plurality of fins (13; 15) are formed on the base member by casting, using a material of good castability, with the base member (11; 21; 36) being an insert. The rotor can be manufactured inexpensively and has the same characteristics as a rotor made from low-carbon-made steel.