1. A perplexity calculation device, comprising: a hardware processor; a weight coefficient calculating unit, implemented on the hardware processor, for, with respect to each of a plurality of text constituent words constituting a text, calculating a weight coefficient for correcting a degree of ease of word appearance having a value which becomes larger as a probability of appearance of the text constituent word becomes higher based on a statistical language model showing probabilities of appearance of words, based on word importance representing a degree of importance of the text constituent word; and a perplexity calculating unit, implemented on the hardware processor, for calculating perplexity of the statistical language model to the text, based on the calculated weight coefficients and the degrees of ease of word appearance.