1. A spatial-multiplexed signal detection method wherein, in a soft-input soft-output detection method in spatial and temporal multiplexed signal separation, a process (factorization) is included for implementing factorization of conditional probability referred to as “likelihood” that is obtained for a signal sequence that is received when a spatial-multiplexed transmission sequence is assumed to have been transmitted such that the conditional probability can be represented as the product of a plurality of conditional probabilities; the conditional probabilities for which factorization is possible are divided into a plurality of groups; when calculating the likelihoods, an ordering can be established among said groups for which probabilities are calculated such that the groups that include events that are the conditions of the conditional probabilities in said groups are processed earlier; and when calculating probabilities in the groups, a metric operation method is used that uses semi-rings for estimating transmission sequences by means of the ratio of the likelihoods of two exclusive events.