1. A speech recognition apparatus comprising: a speech recognizing unit; a speech recognition dictionary storing unit; a conversation database that has stored therein plural pieces of topic specifying information; and a conversation history storing memory that stores subjects included in conversation in the past, wherein: said speech recognizing unit selects plural candidates corresponding to a speech signal on the basis of contents stored in the speech recognition dictionary storing unit, selects a candidate from the selected plural candidates on the basis of topic specifying information associated with conversation histories stored in the conversation history storing memory, and outputs the selected candidate as a recognition result.