A Viterbi decoder decodes a received signal having Ich and Qch components obtained by modulating a convolutional coded multi-valued QAM signal. The Viterbi decoder includes a first unit which inputs the received signal having the Ich and Qch components, a second unit which finds Euclidean distance between a predetermined signal point, which is one of a plurality of predetermined signal points allocated on a plane formed of Ich and Qch axes orthogonally crossed, and a signal point of the received signal, and a third unit which performs a Viterbi decoding based on the Euclidean distance obtained by the second unit.