A gas liquid separator (1A) includes: a sealed container (2) including an upper cover portion (21), a tubular portion (22), and a lower cover portion (23); and three pipes extending from the outside of the sealed container (2) to the inside thereof, that is, a first pipe (3A), a second pipe (3B), and a gas outlet pipe (4). A guide member (5) that forms an inflow space (11) between the guide member (5) and the upper cover portion (21) and forms a flow passage (15) for a two-layer flow F between the guide member (5) and the inner peripheral face of the tubular portion (22) is disposed in the sealed container (2). The first pipe (3A) and the second pipe (3B) are configured such that when one of the first pipe (3A) and the second pipe (3B) is used to introduce a gas-liquid two-phase fluid into the inflow space (11) from outside the sealed container (2), the other one is used to discharge a liquid in a liquid reservoir (13) from the sealed container (2) through a liquid outlet port (31 or 32) of the other one while forming a liquid surface at a level above the liquid outlet port (31 or 32).