1. A signal detection apparatus comprising: first extraction means for extracting a feature amount of an input signal sequence; second extraction means for extracting a feature amount of a noise component contained in the signal sequence; first likelihood calculation means for calculating a first likelihood indicating probability that the signal sequence is detected, on the basis of a predetermined signal-to-noise ratio and the feature amount of the signal sequence extracted by said first extraction means; second likelihood calculation means for calculating a second likelihood indicating probability that the noise component is detected, on the basis of the feature amount of the noise component extracted by said second extraction means; likelihood comparison means for comparing the first likelihood with the second likelihood; and determination means for determining detection of the signal sequence on the basis of a comparison result obtained from said likelihood comparison means.