PURPOSE: To attain high speed decoding processing by approximating square operation for finding out the squared value of a subtracted result value between a received sample value and a predicted sample value by a broken line based upon plural linear functions.

CONSTITUTION: A subtractor 21 subtracts between a received sample value and a predicted sample value and supplies the subtracted result to a broken line conversion circuit 23. The circuit 23 converts the subtracted result value by a broken line approximate function for square operation to obtain a conversion value as a branchmetric value. When the subtracted result value is defined as X, the broken line approximate function is expressed by linear functions Ya, Yb. Namely Ya=2i.X provided with x<t and Yb=(2i.NX-a) provided with X≥t. When the X is smaller than a prescribed value (t), the subtracted result value X is converted into a branchmetric value Y by using the linear function Ya, and when the X is more than the prescribed value (t), the value X is converted into the branchmeteric value Y by using the linear function Yb.