5. The speech synthesis device according to claim 4 , wherein the executable instructions, when executed by said processor, cause said speech synthesis device to further function as a target cost calculation unit configured to, for each of the phonemes generated from the text, select the piece of segment information having the phoneme type that matches the type of the phoneme from among the pieces of segment information stored in the segment storage unit, and calculate a cost indicating agreement between the phoneme environment information of the phoneme and the phoneme environment information included in the selected piece of segment information, wherein the segment selection unit is configured to select, for each of the phonemes generated from the text, the piece of segment information corresponding to the phoneme, based on the degree of agreement and the cost that were calculated for the phoneme.
5. The speech synthesis device according to claim 4 , wherein the executable instructions, when executed by said processor, cause said speech synthesis device to further function as a target cost calculation unit configured to, for each of the phonemes generated from the text, select the piece of segment information having the phoneme type that matches the type of the phoneme from among the pieces of segment information stored in the segment storage unit, and calculate a cost indicating agreement between the phoneme environment information of the phoneme and the phoneme environment information included in the selected piece of segment information, wherein the segment selection unit is configured to select, for each of the phonemes generated from the text, the piece of segment information corresponding to the phoneme, based on the degree of agreement and the cost that were calculated for the phoneme. 6. The speech synthesis device according to claim 5 , wherein the segment selection unit is configured to, for each of the phonemes generated from the text, assign a weight to the cost calculated for the phoneme, and select the piece of segment information corresponding to the phoneme, based on the weighted cost and the degree of agreement calculated by the agreement degree calculation unit, the assigned weight being larger as the pieces of segment information stored in the segment storage unit are larger in number.