1. A maximum likelihood decoder, used as an error correcting decoder provided for reception according to convolutional code and for estimating original data transmitted by calculating the metrics of received coded data, said maximum likelihood decoder comprising: branch metric calculator means for calculating branch metrics out of received encoded data of n.sub.O bits in the channel signal speed and for obtaining the branch metrics of the number k.sub.O ; speed converter means for converting the output from said branch metric calculator means into the signal speed of the original data; and estimating means for estimating original data in correspondence to the coding rate of k.sub.0 /n.sub.1, k.sub.O, n.sub.O, n.sub.l being natural numbers having the relation k.sub.0 &lt;n.sub.0 &lt;N.sub.I.