To simply and accurately achieve the scribing on a log by providing a free slider capable of free movement on a horizontal mounting table, providing a slide rod movable in the vertical direction while keeping the right angle condition relative to the free slider, and carrying a writing material with the slide rod.

When the scribing work is achieved to logs 42, 43, the tip position of each writing material 5 is agreed with each other so that the line connecting the tips of the writing material 5 of upper and lower marking members 4 is perpendicular. At the same time, the distance between the tips of the upper and lower writing materials 5 is agreed with the distance between the centers of the logs 42, 43 to position the upper and lower marking members 4. A free slider 2 is mounted on a frame 50 of a mounting table 41, and a slide rod 1 is operated while the tip of each writing material 5 is abutted on the circumferential side surface of the logs 42, 43, and scribing is achieved on the upper half surface of the circumferential side surface of the logs 42, 43. When the circumferential side surface on the opposite side is scribed, the free slider 2 and the slide rod 1 are transferred to the opposite side.