10. The method according to claim 9 , further comprising: preparing in advance a first storage unit including a plurality of storage mediums with different data acquisition speeds, which store a plurality of speech units, respectively; preparing in advance a second storage unit configured to store information indicating in which one of said plurality of storage mediums each of the speech units is stored; and acquiring the plurality of speech units from the first storage unit in accordance with the information before concatenating the plurality of speech units, and wherein the calculating the penalty coefficient including calculating the penalty coefficient for each of said plurality of third speech unit strings based on a restriction concerning quickness of data acquisition which is to be satisfied when the speech units included in the first speech unit string are acquired from the first storage unit by the concatenation unit and a statistic determined depending on which one of said plurality of storage mediums each of all speech units included in the third speech unit string is stored in.
10. The method according to claim 9 , further comprising: preparing in advance a first storage unit including a plurality of storage mediums with different data acquisition speeds, which store a plurality of speech units, respectively; preparing in advance a second storage unit configured to store information indicating in which one of said plurality of storage mediums each of the speech units is stored; and acquiring the plurality of speech units from the first storage unit in accordance with the information before concatenating the plurality of speech units, and wherein the calculating the penalty coefficient including calculating the penalty coefficient for each of said plurality of third speech unit strings based on a restriction concerning quickness of data acquisition which is to be satisfied when the speech units included in the first speech unit string are acquired from the first storage unit by the concatenation unit and a statistic determined depending on which one of said plurality of storage mediums each of all speech units included in the third speech unit string is stored in. 12. The method according to claim 10 , wherein said plurality of storage mediums include a storage medium with a high data acquisition speed and a storage medium with a low data acquisition speed, and the restriction is an upper limit value of a time required to acquire all speech unit data included in the first speech unit string from the first storage unit, and the statistic is a predictive value of a time required to acquire all speech unit data included in the third speech unit string from the first storage unit.