An observation system includes an illumination unit including a light source from which emits an illumination light including light of a plurality of colors, and light receiving unit including an optical sensor which acquires a light intensity. The illumination unit emits the illumination light for a culture vessel so that the illumination light travels from a first side to a second side of the culture vessel, the first side and the second side being defined by interposing a culture medium. The light receiving unit receives, on the first side, light emitted from the illumination unit, entering the culture vessel from the first side, reflected at the second side, and transmitted through the culture medium, and acquires a light intensity for each color in the received light for calculation of a pH value of the culture medium.