1. A voice recognition system which recognizes keywords included in input voice, comprising: a first voice recognition processing means for generating a recognition hypothesis graph which indicates a structure of hypothesis that is derived according to a first grammar together with a score associated with respective connections of a recognition unit by executing a voice recognition process based on said first grammar to a voice feature amount of input voice; and a second voice recognition processing means for outputting a recognition result from total score of a hypothesis which is derived according to a second grammar after acquiring said structure and said score of said garbage section from said recognition hypothesis graph, and executing a voice recognition process according to said second grammar that is specified to accept a section other than keywords in input voice as a garbage section to a voice feature amount of input voice, wherein said second voice recognition processing means, to said hypothesis reached to said garbage section in said second grammar, selects no smaller than one voice section having a node which can connected with said hypothesis as a starting point in said recognition hypothesis graph, and connects said structure and said score of a selected voice section with said hypothesis as said structure and said score of said garbage section.