8. The operation assisting device according to claim 3 , wherein the keyword detector includes a keyword determiner that calculates an evaluation value indicating a likelihood that the voice data match with the preliminarily stored keywords, and determines that the voice data match with the preliminarily stored keywords, in a case where the calculated evaluation value is greater than or equal to a first threshold value; an information storage that stores therein the voice data corresponding to the evaluation value, in a case where the evaluation value is less than the first threshold value and greater than or equal to a second threshold value; and an utterance determiner that calculates the similarity between or among the voice data in a case where a predetermined number of the voice data is, input during a predetermined time period before a timing when detecting that the sight lines of the speaker are directed at the operation target, are stored in the information storage, the utterance determiner determining that the voice data match with the preliminarily stored keywords, under a condition that the calculated similarity falls within the predetermined range.
8. The operation assisting device according to claim 3 , wherein the keyword detector includes a keyword determiner that calculates an evaluation value indicating a likelihood that the voice data match with the preliminarily stored keywords, and determines that the voice data match with the preliminarily stored keywords, in a case where the calculated evaluation value is greater than or equal to a first threshold value; an information storage that stores therein the voice data corresponding to the evaluation value, in a case where the evaluation value is less than the first threshold value and greater than or equal to a second threshold value; and an utterance determiner that calculates the similarity between or among the voice data in a case where a predetermined number of the voice data is, input during a predetermined time period before a timing when detecting that the sight lines of the speaker are directed at the operation target, are stored in the information storage, the utterance determiner determining that the voice data match with the preliminarily stored keywords, under a condition that the calculated similarity falls within the predetermined range. 9. The operation assisting device according to claim 8 , wherein the utterance determiner inquires of the speaker about whether or not the voice data match with the preliminarily stored keywords, and determines that the voice data match with the preliminarily stored keywords, in a case where an asnswer informing that the voice data match with the preliminarily stored keywords is input and the similarity falls within the predetermined range.