An observation system (100, 101) comprising: a surgical microscope; an OCT unit adapted to acquire an OCT signal; and a processor, wherein the surgical microscope includes: an observation optical system adapted to optically guide an observation luminous flux from a biological object being an observation object; and an observation focus adjustment unit provided on an optical path of the observation luminous flux in the observation optical system, the observation focus adjustment unit being adapted to adjust a focus of the observation optical system, the OCT unit includes: an OCT light source; a light splitter adapted to split a luminous flux emitted from the OCT light source into a measurement luminous flux and a reference luminous flux; a photodetection element adapted to receive an interference light obtained by synthesis of the reference luminous flux and a measurement luminous flux reflected by the biological object; and an optical path length difference adjustment unit provided on at least one of an optical path of the measurement luminous flux and an optical path of the reference luminous flux, the optical path length difference adjustment unit being adapted to adjust an optical path length difference between the measurement luminous flux and the reference luminous flux, the processor is adapted to acquire a focus state of the observation optical system and to drive the optical path length difference adjustment unit of the OCT unit in accordance with a change in a focus state of the observation optical system, the OCT unit further includes: an OCT focus adjustment unit provided on the optical path of the measurement luminous flux, the OCT focus adjustment unit being adapted to adjust a focus of an optical system that optically guides the measurement luminous flux, characterised in that the processor is adapted to drive the OCT focus adjustment unit in accordance with a change in the focus state of the observation optical system. a surgical microscope; an OCT unit adapted to acquire an OCT signal; and a processor, wherein the surgical microscope includes: an observation optical system adapted to optically guide an observation luminous flux from a biological object being an observation object; and an observation focus adjustment unit provided on an optical path of the observation luminous flux in the observation optical system, the observation focus adjustment unit being adapted to adjust a focus of the observation optical system, an observation optical system adapted to optically guide an observation luminous flux from a biological object being an observation object; and an observation focus adjustment unit provided on an optical path of the observation luminous flux in the observation optical system, the observation focus adjustment unit being adapted to adjust a focus of the observation optical system, the OCT unit includes: an OCT light source; a light splitter adapted to split a luminous flux emitted from the OCT light source into a measurement luminous flux and a reference luminous flux; a photodetection element adapted to receive an interference light obtained by synthesis of the reference luminous flux and a measurement luminous flux reflected by the biological object; and an optical path length difference adjustment unit provided on at least one of an optical path of the measurement luminous flux and an optical path of the reference luminous flux, the optical path length difference adjustment unit being adapted to adjust an optical path length difference between the measurement luminous flux and the reference luminous flux, an OCT light source; a light splitter adapted to split a luminous flux emitted from the OCT light source into a measurement luminous flux and a reference luminous flux; a photodetection element adapted to receive an interference light obtained by synthesis of the reference luminous flux and a measurement luminous flux reflected by the biological object; and an optical path length difference adjustment unit provided on at least one of an optical path of the measurement luminous flux and an optical path of the reference luminous flux, the optical path length difference adjustment unit being adapted to adjust an optical path length difference between the measurement luminous flux and the reference luminous flux, the processor is adapted to acquire a focus state of the observation optical system and to drive the optical path length difference adjustment unit of the OCT unit in accordance with a change in a focus state of the observation optical system, the OCT unit further includes: an OCT focus adjustment unit provided on the optical path of the measurement luminous flux, the OCT focus adjustment unit being adapted to adjust a focus of an optical system that optically guides the measurement luminous flux, characterised in that the processor is adapted to drive the OCT focus adjustment unit in accordance with a change in the focus state of the observation optical system.