The present invention provides a pulse oximeter of the portable type, which is possible to carry around and to use widely regardless of the adult, the infant, the newborn baby, and which can keep the finger still for performing the precise measurement. The pulse oximeter 100 comprises a measurement cavity 110 for accepting the examined portion (for example finger) of the subject, a measurement device 120 including a light emitting part 121 and a photo detecting part 122 facing between the measurement cavity 110, a filling element 140 formed from a material that the light used for the measurement can penetrate. The height and the curvature of the inner surface of the filling element 140 is larger than that of the outer surface of the filling element 140. The filling element 140 converts the height and the curvature of the inner space. The filling element 140 fills the gap space between the examined portion of the subject and the inner space of the measurement cavity 110, wherein the examined portion of the subject is pressed by the filling element 140 so as not to move freely in the measurement cavity.
