1. An endoscope inserting direction detecting method comprising: a first step of receiving an endoscopic image; a second step of sampling pixels, which represent high densities, from data of the endoscopic image; a third step of defining an approximate expression for providing an approximate state of a distribution of sampling-pixels; a fourth step of evaluating a difference between a state of distribution of the sampling-pixels and a result of an approximation based on the approximate expression; and a fifth step of determining an endoscope inserting direction, in which an endoscope should be inserted, on the basis of a result of an evaluation of the fourth step.