A plural number of switches which are selectively opened and closed comprise a first switch corresponding with the weight "2/3" in APEX value and a second switch corresponding with the weight "1/3" in APEX value while a n bit register comprises a first bit with the weight "1/2" in APEX value, a second bit with the weight "1/4" in APEX value and a third bit with the weight "1/8" in APEX value. The closing signal of the first switch is put in the first bit, the closing signal of the second switch in the second bit and the closing signal of the third switch in the third bit so as to carry out the approximate conversion of the data.