The present invention relates to an insert for slot milling, and more specifically, to an insert for slot milling which can smoothly form a front end surface of a concave portion of a slot formed at a cutting target, and provides a safe use thereof by preventing the erroneous installation of cutting inserts coupled with insert-installation seats on both sides of a milling cutter body to prevent damage of the cutting inserts and the milling cutter. To this end, the present invention is characterized in that the plurality of cutting inserts are provided around the slot milling cutter body, one of two cutting inserts protrudes with respect to one side of the milling cutter body and the other protrudes with respect to the opposite side of the milling cutter body, and two cutting inserts are formed with different structures to prevent the erroneous installation thereof on the insert-installation seats prepared on both sides of the milling cutter body.