PURPOSE: To form a circle or circular arc having a desired size by mounting a means estimating the interval between the leading ends of two shafts corresponding to the opening degree of two shafts constituting compasses and a means for displaying said interval.

CONSTITUTION: When a circle or circular arc is formed, two shafts 2, 3 of compasses are opened at first. The radius of the circle or circular arc capable of being formed at that time is displayed on an LCD display body 6 according to the opening degree of two shafts and can be determined. When a reset button is pushed when the compasses are closed, the distance between two shafts at that time becomes a reference value. Therefore, at each time when the reset button is pushed, the radius of the circle or circular arc is calibrated and can be determined. A pencil 5 wears out with use and an error is generated but the tip position of the lead of the pencil is optically detected to move the pencil up and down so as to set the difference with a reference position to zero.