1. A search device comprising: a memory device including a memory area which stores document data containing search target character strings to which delimiting characters are appended at both ends; an acquirer which acquires a keyword; a generator which generates a search character string by appending delimiting characters to both ends of the keyword acquired by the acquirer; a designator which extracts multiple N-gram strings from the search character string generated by the generator and executes a designation process to designate appearance positions where those extracted N-gram strings appear in one of the search target character strings of the document data; a selector for selecting an N-gram contained in the search target character string one at a time; a counter for counting a number of N-grams whose relative positions match when the search target character string and the search character string are compared, based on a position where the selected single N-gram appears, for each single N-gram selected by the selector; an evaluator which evaluates a degree of similarity between the search target character string and the search character string based on a frequency of a count value counted by the counter and a difference in character lengths of the search target character string and the search character string; and an output device which outputs the search target character string based on evaluation results of the evaluator.