To provide a Viterbi decoding circuit that can prevent overflow of path metrics, without increasing the latency of the circuit.

An ACS circuit is provided with a subtractor circuit, an alarm signal is given to the subtractor circuit when a main comparator circuit detects that each path metric is a predetermined threshold or larger, and each ACS circuit subtracts a predetermined decreased value from the path metric, on the basis of the alarm signal to conduct subtractor processing in the clock of the same ACS processing.