There is provided a reference leak generating device capable of precisely generating an ultra-fine reference leak. The reference leak generating device adapted to be connected to an upstream side of a measurement chamber includes a chamber connected to the measurement chamber through an orifice or a porous plug having a molecular flow conductance C and a pressure to establish molecular flow conditions which are known in advance, and is characterized in that a pressure p1 of testing gas to be introduced into the chamber is precisely determined by using a static expansion method once or more times, and a leak rate of a reference leak at the pressure p1 is obtained in accordance with a product of C and p1.