1. An operation assisting method comprising: comparing voice data with preliminarily stored keywords associated with an operation target and determining whether the voice data match with the preliminarily stored keywords or not; determining whether or not one or more of the preliminarily stored keywords are included in the voice data, and whether or not a similarity between or among the voice data falls within a predetermined range; detecting whether or not sight lines of a speaker are directed at the operation target only when it is determined that the voice data do not match with the preliminarily stored keywords, and the similarity between or among the voice data falls within the predetermined range; and determining that the one or more of the preliminarily stored keywords are included in the voice data only when the similarity between or among the voice data are within the predetermined range during a predetermined time period before a timing when the sight lines of the speaker are directed at the operation target.