An optical instrument for measuring a total reflection absorption spectrum condensing an infrared light on a sample-side surface of an ATR crystal (4, 104) configured to come into contact with a sample, and to be used to measure an absorption spectrum of a total reflection light from the sample-side surface, comprising: the ATR crystal (4, 104) having the sample-side surface configured to come into contact with the sample; an infrared light condensing means (20, 120) on an incident side configured to guide the infrared light to condense at least a portion of the infrared light to the sample-side surface at an incident angle equal to or larger than a critical angle θc inside of the ATR crystal (4, 104); an infrared light condensing means (22, 122) on an exiting side configured to condense the total reflection light from the sample-side surface; and an illumination light guiding means (18, 24) configured to guide a visible light coaxially to an optical axis of at least one of the infrared light condensing means on the incident side and the exiting side (20, 22, 120, 122); wherein the optical instrument additionally comprises: a visible light condensing means (40, 42, 44) configured to condense the visible light from the sample-side surface to observe a contact state of the sample from a direction different from the respective optical axes of the infrared light condensing means (20, 22, 120, 122) on the incident side and the exiting side; wherein the illumination light guiding means (18, 24) is configured to guide the visible light to the infrared light condensing means (20, 22, 120, 122) so that at least a portion of the visible light is condensed to the sample-side surface at an incident angle less than a critical angle θc' of the visible light characterized in that the visible light condensing means (40, 42, 44) is configured to condense the visible light along the optical axis perpendicular to the sample-side surface. the ATR crystal (4, 104) having the sample-side surface configured to come into contact with the sample; an infrared light condensing means (20, 120) on an incident side configured to guide the infrared light to condense at least a portion of the infrared light to the sample-side surface at an incident angle equal to or larger than a critical angle θc inside of the ATR crystal (4, 104); an infrared light condensing means (22, 122) on an exiting side configured to condense the total reflection light from the sample-side surface; and an illumination light guiding means (18, 24) configured to guide a visible light coaxially to an optical axis of at least one of the infrared light condensing means on the incident side and the exiting side (20, 22, 120, 122); wherein the optical instrument additionally comprises: a visible light condensing means (40, 42, 44) configured to condense the visible light from the sample-side surface to observe a contact state of the sample from a direction different from the respective optical axes of the infrared light condensing means (20, 22, 120, 122) on the incident side and the exiting side; wherein the illumination light guiding means (18, 24) is configured to guide the visible light to the infrared light condensing means (20, 22, 120, 122) so that at least a portion of the visible light is condensed to the sample-side surface at an incident angle less than a critical angle θc' of the visible light characterized in that the visible light condensing means (40, 42, 44) is configured to condense the visible light along the optical axis perpendicular to the sample-side surface. a visible light condensing means (40, 42, 44) configured to condense the visible light from the sample-side surface to observe a contact state of the sample from a direction different from the respective optical axes of the infrared light condensing means (20, 22, 120, 122) on the incident side and the exiting side; wherein the illumination light guiding means (18, 24) is configured to guide the visible light to the infrared light condensing means (20, 22, 120, 122) so that at least a portion of the visible light is condensed to the sample-side surface at an incident angle less than a critical angle θc' of the visible light characterized in that the visible light condensing means (40, 42, 44) is configured to condense the visible light along the optical axis perpendicular to the sample-side surface.