PURPOSE: To eliminate the troublesomeness for always confirming a position of a decimal point in the course of calculation, and to execute a calculation more quickly and more exactly by providing a movable comma/decimal point projection.

CONSTITUTION: A groove 3 is hollowed out on a counter stop plate 2 of an abacus. Subsequently, a comma/decimal point display bar 4 which consists of the same shape as the groove 3 and is a little smaller than it is provided, and also a comma projection 5 and a decimal point projection 6 are provided on the display bar 4. In this state, when executing a multiplication or a division containing a decimal point, the decimal point projection 6 is moved by a fingertip and aligned with a decimal point position where an answer appears on the abacus. Thereafter, an answer of the calculation is found and a decimal point position and a comma position are confirmed. In this way, the troublesomeness for always confirming continuously a position of a decimal point when counters of the abacus are being moved is eliminated, it is possible to concentrate on the calculation by that portion, and the calculation can be executed more quickly and more exactly.