To provide a groove forming device capable of preventing occurrence of a problem such as breaking, cracks, or burrs at both ends of a groove formed in an object.

This groove forming device 1M forms a groove 3 extended from one end surface 4 abutting on the opening surface 2a of a sheet 2 to the other end surface 5 abutting on the opening surface 2a. It is provided with a first rotary blade 35 retractable in a direction separating from the opening surface 2a of the sheet 2 while having its cutting edge 35a formed in its outer peripheral section rotated in a direction toward the one end surface 4 of the sheet 2 to cut a section from the one end surface 4 of the sheet 2 to the middle section of the opening surface 2a of the sheet 2 in the groove 3, and a second rotary blade 71 retractable in the direction separating from the opening surface 2a while having its cutting edge 71a formed in its outer peripheral section rotated in a direction toward the other end surface 5 of the sheet 2 to cut a section from the other end surface 5 of the sheet 2 to the middle section of the opening surface 2a in the groove 3.