A substrate exposing apparatus includes an immersion exposure unit, disposed between a projection optical system and a substrate, including a vessel disposed on an optical path and filled with a liquid, a supply line connected to one side of the vessel to supply the liquid to the vessel, a first drain line connected to the other side of the vessel to drain the liquid from the vessel, and a monitoring unit including at least one first measuring unit connected to the first drain line to detect a property of the liquid flowing through the first drain line. The monitoring unit can include a collection line connected to the first drain line to collect the liquid, a first bath storing the collected liquid, and a first distribution line through which the liquid in the first bath can flow. The first measuring unit is installed on the first distribution line.