A flow channel structure comprising: a substrate including a supply flow channel that guides a measurement target liquid toward the inside of the substrate; a separation element accommodating unit that accommodates a separation element that separates a plurality of components included in the measurement target liquid, the measurement target liquid being guided by the supply flow channel ; and a detection unit that guides the measurement target liquid that passes through the separation element accommodating unit , wherein measuring light for measuring information about the plurality of components is to be irradiated onto the measurement target liquid, wherein the detection unit includes a measurement flow channel part that guides the measurement target liquid, an incident part that is provided at an end of the measurement flow channel part and that guides the measuring light toward inside the measurement flow channel part , and an emission part that is provided at the other end of the measurement flow channel part and that derives the measuring light from the measurement flow channel part ; characterized in that the emission part includes a reflection part that is provided at a position opposite to the incident part and that reflects the measuring light irradiated from the incident part onto the measurement target liquid and through the incident part . a substrate including a supply flow channel that guides a measurement target liquid toward the inside of the substrate; a separation element accommodating unit that accommodates a separation element that separates a plurality of components included in the measurement target liquid, the measurement target liquid being guided by the supply flow channel ; and a detection unit that guides the measurement target liquid that passes through the separation element accommodating unit , wherein measuring light for measuring information about the plurality of components is to be irradiated onto the measurement target liquid, a supply flow channel that guides a measurement target liquid toward the inside of the substrate; a separation element accommodating unit that accommodates a separation element that separates a plurality of components included in the measurement target liquid, the measurement target liquid being guided by the supply flow channel ; and a detection unit that guides the measurement target liquid that passes through the separation element accommodating unit , wherein measuring light for measuring information about the plurality of components is to be irradiated onto the measurement target liquid, wherein the detection unit includes a measurement flow channel part that guides the measurement target liquid, an incident part that is provided at an end of the measurement flow channel part and that guides the measuring light toward inside the measurement flow channel part , and an emission part that is provided at the other end of the measurement flow channel part and that derives the measuring light from the measurement flow channel part ; characterized in that the emission part includes a reflection part that is provided at a position opposite to the incident part and that reflects the measuring light irradiated from the incident part onto the measurement target liquid and through the incident part . a measurement flow channel part that guides the measurement target liquid, an incident part that is provided at an end of the measurement flow channel part and that guides the measuring light toward inside the measurement flow channel part , and an emission part that is provided at the other end of the measurement flow channel part and that derives the measuring light from the measurement flow channel part ; characterized in that the emission part includes a reflection part that is provided at a position opposite to the incident part and that reflects the measuring light irradiated from the incident part onto the measurement target liquid and through the incident part . <EOS>