PURPOSE: To provide a number of significant digits detector for quickly finding out the number of significant digits of a binary number.

CONSTITUTION: The number of significant digits detector 1 is constituted of a coincidence circuit string 12, a zero priority circuit 14, an exclusive OR circuit string 16, an encoder 18, or the like. The circuit string 12 is a combined circuit for judging the coincidence/noncoincidence of adjacent bits in a binary number. The circuit 14 is a combined circuit for setting up all values of lower bits lower than a position whose bit value is changed at first at the time of counting it from the most significant position out of the outputs of the circuit string 12 to the same value as that of the bit arranged on the position. The circuit string 16 is a combined circuit for deciding the coincidence/noncoincidence of adjacent bits in an output from the circuit 14. The encoder 18 is a combined circuit for converting the position of a bit whose value is different from the value of other bits into a binary number.