A flow volume measurement device or a flow velocity measurement device include a measurement cell including a main pipe, an incident tube that is connected to the main pipe, an emission tube that is connected to the main pipe, and a first purge-fluid supply tube that is connected to the incident tube, a purge-fluid supply unit that supplies purge fluid into the first purge-fluid supply tube of the measurement cell, a light emitting unit that emits a laser beam to the measurement cell, a light receiving unit that receives the laser beam emitted from the light emitting unit and having passed through the measurement cell, and outputs a received amount of light as a light reception signal, a calculation unit that calculates a flow volume or a flow velocity of exhaust fluid flowing in the measurement cell, based on a light reception signal output from the light receiving unit.