PURPOSE: To attain a calculation method of simple constitution with a small number of conversion tables by performing the logarithm conversion after executing the sequential algorithm by the program of a digital signal processor.

CONSTITUTION: When the logarithm conversion (Y=logaX) is carried out, a window is provided to a prescribed bit of the Y information consisting of plural bits. Then the data on the X information is supplied to the 1st register and the initial value of the X information is supplied to the 2nd register. A coeffi cient pointer related to the (a) information is initialized. The coefficient corre sponding to said coefficient point and the multiplied value of the holding value of the X information are compared with data. If the multiplied value is larger than the data, this multiplied value is supplied to the 2nd register in place of the holding value of the X information. Then a specific bit is set up at a prescribed bit of the Y information. While the window is shifted and at the same time the coefficient is changed for repetition of said comparison in case the multiplied value is smaller than the data.