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, wherein said likelihood comparison means compares the first likelihood with the second likelihood in accordance with: L f = ∏ k = 1 S ⁢ 1 ⅇ ω ⁢ γ ⁡ ( 0 , ω ) ⁢ ( s k 2 μ k + ω ) ⁡ [ exp ⁡ ( s k 2 μ k + ω ) - 1 ] where L f represents a likelihood ratio in a frame f, s 2 k represents a kth element of a spectral power vector serving as the feature amount of the signal sequence extracted by said first extraction means in the frame f, μ k represents a kth element of a noise estimation vector serving as the feature amount of the noise component extracted by said second extraction means in the frame f, S represents the number of vector elements, ω represents the signal-to-noise ratio, and γ represents a lower incomplete gamma function.