1. An information processing apparatus comprising: a setting unit that sets a search character string; a searching unit that searches information including the set search character string; and a determining unit that extracts a co-occurrence character string candidate group among all partial character strings appearing in a plurality of pieces of the information obtained as a search result, wherein the determining unit determines a co-occurrence character string from the co-occurrence character string candidate group, wherein the setting unit sets one or more contrast character strings to characterize the search character string, wherein the one or more contrast character strings is in contrast with the search character string, and wherein the determining unit determines the co-occurrence character string, on the basis of the comparison result of the appearance frequency of each co-occurrence character string candidate forming the co-occurrence character string candidate, group in the information including the search character string and the appearance frequency of each co-occurrence character string candidate in information including the contrast character string.