A groove cutter for floor sheets capable of being operated without using a ruler and can cut a seam, which is created when sheets are laid sequentially on the surface of floor base with an adhesive agent, so that a groove formed along the seam can bridge over both adjacent sheets, comprising a front part, a middle part, a rear part, wherein the front part has a thin guide plate, which is to be inserted into the seam and guides the tool along the seam when pushing the tool forward, at the front end, and at least one blade to cut a concave groove at the seam part of the sheet, which is provided behind the guide plate so as to be detachable, and a stabilizing member provided at the rear part, which is to be inserted into the concave groove, to stabilize a direction of movement along the seam.