PURPOSE: To reduce the rate of the number of ROMs contained in the computer hardware by carrying out selectively the operations to add '1' to the output contents of a constant generating means based on the result of comparison of a comparator means.

CONSTITUTION: A comparator 2 stores and compares the output value (k) of an exponent computing element 1 with a prescribed constant. The finction values are sequentially generated as the constants in a range covering K=0 through k=m-1 in rK=2k × arctan (2-k) where k=0, 1...m-1 and (m) is equal to an integer. At the same time, the values of 1/K are sequentially generated as constants in a range covering k=0 through k=m-1 in the function rK=IIm (1+1-2k)1/2 where k=0, 1...m-1 and (m) is a positive integer. The constant values of 1/k are selectively outputted based on the result of comparison of the comparator 2. Then an adder 6 selects the addresses of rK in response to the result of comparison of the comparator 2 and increases these addresses. An adder 10 adds selectively '1' to the most significant bit MSB of the output contents of the function rK based on the result of comparison of the comparator 2. As a result, the occupying rates of the constant ROM 3 and 4 are reduced in the computer hardware.