To surely and easily replaceably attach parts to the luminaire body of a luminaire.

This luminaire ballast 11 for attaching to a luminaire body 3 is provided with a ballast body 15 and a terminal block part 31. The terminal block part 31 is mounted from above, on a terminal block mounting part 21 provided at the ballast body 15. Connection 39, 24 provided respectively at the terminal block part 31 and the terminal block mounting part 21 engage each other, whereby the terminal block part 31 and the terminal block mounting part 21 can be electrically connected together and positioned. When the luminaire ballast 11 is fixed to the luminaire body 3 with the use of a screw 43, a nut 44, and a washer 36, the washer 36 breaks paint on the luminaire body 3 and makes contact with a metallic part, enabling the luminaire body 3 to be grounded to a wire 42 via the screw 43 and the terminal block part 31.