1. A voice recognition system which recognizes keywords included in input voice, comprising: a first voice recognition processing unit that generates 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 unit that outputs a recognition result from total score of a hypothesis which is derived according to a second grammar after 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 unit acquires said structure and said score of said garbage section from said recognition hypothesis graph.