A detection apparatus which illuminates a sample and detects light reflected by the sample, comprises a light source, a columnar reflecting member having a columnar reflecting surface which reflects light having entered a first end of the columnar reflecting member by a plurality of number of times, and emits the light from a second end of the columnar reflecting member, a mirror which reflects light radiated by the light source so as to guide to the first end and a detector, wherein the sample is illuminated with the light emitted from the second end, and the detector is configured to detect the light which has been reflected by the sample and has passed through the columnar reflecting member, and a reflecting surface of the mirror is a concave surface, and a shape of a reflecting surface of the mirror on a section perpendicular to an axis of the columnar reflecting member is concave.