PURPOSE: To attain a statistical code book capable of obtaining excellent reproduced sound quality even in a low bit rate.

CONSTITUTION: The statistical code book 70 is provided with two 1st and 2nd statistical code book parts 71, 72 capable of simultaneously outputting plural statistical code vectors CBa, CBb. Both outputted code vectors weighted and added by respective weighted addition parts (73 to 75) and outputted as an output vector of the statistical code book 70. A weighting coefficient determining part 76 determines weighting coefficients α, 1-α based upon voice vectors already synthesized based upon the outputted stastistical code vector or the information of the input voice vector. Consequently reproduced sound quality similar to a case using many statistical code vectors can be obtained by the use of the smaller number of code vectors and information to be transmitted/ received by a coder and a decoder can be reduced.