PURPOSE: To provide an abacus entering a (2n+1)-adic having not been calculated by a conventional abacus and being a new notation or capable of calculation of the (2n+1)-adic operation.

CONSTITUTION: The abacus 1 is provided with a positive region in which positive integers (1-n) are represented depending on the number of beads 3 placed to each digit, a negative region in which negative integers (-1 to -n) are represented depending on the number of beads 3 placed to each reed, and a zero region where only zero is represented independently of the number of beads 3. Then any of integers -n to n is represented based on the beads 3 placed on each reed, and the (n: a positive integer being 1 or over) can represents (2n+1)-adic number in 1 to plural reeds.