PURPOSE: To provide a portable electric drill unit by which drilling a slanting hole to a board thickness face of a plate material is prevented and positioning of a hole machining position is carried out easily while easy portableness is accomplished.

CONSTITUTION: An attachment for an electric drill consists of a clamp part 3 arranged in the front part 2 of a connecting frame 1 and a drill mounting part 5 arranged in the rear part, and the connecting frame 1 is provided with an inserting port 6 so that a drill edge part is inserted forward and backward freely, while the clamp part 3 consists of clamp plates 11, 11, which are situated in the front part, 2 of the connecting frame 1 and one of them or both of them are elevated freely via a guide post, and a fastening screw 12 elevating the clamp plates. The drill mounting part 5 consists of traveling guides 13, 13 extended to the rear side of the connecting frame 1, a mounting plate 14 arranged freely travelably in the traveling guides, and a drawing spring 15 connecting the mounting plate 14 and the rear parts of the traveling guides 13 together.