1. A similarity degree calculation method for use in calculating a similarity degree between a plurality of texts by a computer system including a process, a storage unit an input device and an output device, the method comprising: storing the plurality of texts via the input device into the storage unit; extracting, by the processor, each common character string that has a predetermined character string length and that is sent from the plurality of texts to the storage unit; determining, by the processor, whether or not two texts of the plurality of texts are to be set a Kolmogorov complexity calculation objects based on a number of the extracted each common character strings and a predetermined number of common character strings, as a result of pretreatments of each similarity degree calculation of the plurality of texts by using the Kolmogorov complexity, wherein a pretreatment of the pretreatments refers to the number of extracted common character strings in an original language and determines whether a processing of the Kolmogorov complexity to the similarity degree calculation is to be performed, before performing a calculation processing of the Kolmogorov complexity between the two texts; by the processor, each similarity degree of the plurality of texts, when the two texts are the Kolmogorov complexity calculation objects, calculating the similarity degree of a Kolmogorov complexity, and when the two texts are not the Kolmogorov complexity calculation objects, handing the similarity degree between the stored two texts in the storage unit as being dissimilar; and outputting, by the processor and the output device, the each similarity degree between the stored two texts in the storage unit.