1. A navigation device, comprising: a storage means, which stores street names; a first street name receiving means, which receives voice input of a whole street name; a first street name specifying means, which specifies a street name that best matches the voice received by the first street name receiving means among the street names stored in the storage means; a confirmation means, which receives confirmation of the street name specified by the first street name specifying means; a letter receiving means, which receives voice input of at least one letter from a character string expressing the street name specified by the first street name specifying means, when the confirmation means has not received confirmation of the specified street name; a letter specifying means, which recognizes the voice input received by the letter receiving means, so as to specify a letter; an extraction means, which extracts candidate street names from the street names stored in the storage means, by using the letter specified by the letter specifying means; a second street name receiving means, which receives voice input of the whole street name after the letter receiving means receives the voice input; a second street name specifying means, which specifies a street name that best matches the voice input received by the second street name receiving means, from among the street names extracted by the extraction means; and an output means, which outputs the street name specified by the second street name specifying means.