PURPOSE: To generate an approximation function capable of calculating the limited value of a dependent variable suited to a purpose without the need of deciding other values of the approximation function and to provide the value capable of approximation with a controlled maximum error by using a limited hardware means.

CONSTITUTION: This device is provided with a first means 10 for repeatedly deciding at least one present linear regression function by the fact that absolute values measured among the first values Y3, Y4 and Y5 of dependent variables of a series of three pairs X3, Y3; X4, Y4; and X5, Y5 of first pairs and the second values Y'3, Y'4 and Y'5 of dependent variables decided corresponding to the present linear regression function for the first values X3, X4 and X5 of independent variables are equal and codes form alternate first errors, selecting one of the present linear regression functions for generating the approximation of all of the series of the pairs provided with a minimum error and coding the selected linear regression function by specified codes (p) and (q) and a second means 17 for deciding a second pair XA, Y'A corresponding to the specified codes.