The proposed flush siphon comprises a housing (1) with an inlet opening (3) and an outflow opening (4); and an L-shaped immersion pipe (7) which can be fitted in the housing (1). An approximately horizontal inlet pipe section (8) of the immersion pipe (7) discharges into an approximately vertical immersion pipe section (9) of the immersion pipe (7) from the inlet opening on a side wall of the housing (1); the inlet pipe section (8) can be connected as a duct by a sealed joint to a drainage pipe (15) such as a sink unit drainage pipe; and the outflow opening (4) of the housing (1) discharges into an outflow channel. To facilitate particularly simple cleaning of the concealed siphon and ensure ease of access to the outflow channel, the immersion pipe section (9) of the immersion pipe (7) is formed as an integral part of a rising pipe section (10) in the manner of communicating vessels to create a siphon.