A small reactor, which contains an inorganic transparent substrate, which contains: a reaction channel configured to allow a chemical reaction to proceed therein; a supply channel, which is connected to one end of the reaction channel, and is configured to supply samples to be reacted in the reaction channel; and a discharge channel, which is connected to the other end of the reaction channel, and is configured to discharge a reaction product from the reaction channel, wherein the inorganic transparent substrate is in the shape of an arc-shaped curve.