A sound signal analysis apparatus comprising: sound signal input means adapted for inputting a sound signal indicative of a musical piece; tempo detection means adapted for detecting a tempo of each of time serial sections of the musical piece by use of the input sound signal; judgment means adapted for judging stability of the tempo; and control means adapted for controlling a certain target apparatus (EXT, 16) in accordance with a result judged by the judgment means, characterized in that the tempo detection means has: feature value calculation means adapted for calculating a first feature value indicative of a feature relating to existence of a beat and a second feature value indicative of a feature relating to tempo for each of the time serial sections of the musical piece; and estimation means adapted for concurrently estimating a beat position and a change in tempo in the musical piece by selecting, from among a plurality of Hidden Markov Models described as sequences of states (qb, n) classified according to a combination of a physical quantity (n) relating to existence of a beat in each of the time serial sections and a physical quantity (b) relating to tempo in each of the time serial sections, a probability model whose sequence of observation likelihoods each indicative of a probability of concurrent observation of the first feature value and the second feature value (Xb) in the respective section of the musical piece satisfies a certain criterion. sound signal input means adapted for inputting a sound signal indicative of a musical piece; tempo detection means adapted for detecting a tempo of each of time serial sections of the musical piece by use of the input sound signal; judgment means adapted for judging stability of the tempo; and control means adapted for controlling a certain target apparatus (EXT, 16) in accordance with a result judged by the judgment means, characterized in that the tempo detection means has: feature value calculation means adapted for calculating a first feature value indicative of a feature relating to existence of a beat and a second feature value indicative of a feature relating to tempo for each of the time serial sections of the musical piece; and estimation means adapted for concurrently estimating a beat position and a change in tempo in the musical piece by selecting, from among a plurality of Hidden Markov Models described as sequences of states (qb, n) classified according to a combination of a physical quantity (n) relating to existence of a beat in each of the time serial sections and a physical quantity (b) relating to tempo in each of the time serial sections, a probability model whose sequence of observation likelihoods each indicative of a probability of concurrent observation of the first feature value and the second feature value (Xb) in the respective section of the musical piece satisfies a certain criterion. feature value calculation means adapted for calculating a first feature value indicative of a feature relating to existence of a beat and a second feature value indicative of a feature relating to tempo for each of the time serial sections of the musical piece; and estimation means adapted for concurrently estimating a beat position and a change in tempo in the musical piece by selecting, from among a plurality of Hidden Markov Models described as sequences of states (qb, n) classified according to a combination of a physical quantity (n) relating to existence of a beat in each of the time serial sections and a physical quantity (b) relating to tempo in each of the time serial sections, a probability model whose sequence of observation likelihoods each indicative of a probability of concurrent observation of the first feature value and the second feature value (Xb) in the respective section of the musical piece satisfies a certain criterion.