To provide a siphon type drainage system with excellent durability, which comprises a drain standpipe, a drain pipe arranged from appliances in a water section on each story and a combined joint for connecting the drain pipe to the standpipe, which makes the drain pipe merge with the standpipe by making the level of the combined joint lower than that of a drainage story floor slab, and which facilitates installation operations; and to provide a flexible pipe member for use in the drainage system.

Characteristically, a rigid vertical piping line, connected to the combined joint, of the drain pipe of the siphon type drainage system is partially formed as a flexible pipe body; and a rigid cover capable of being attached to/detached from an outer periphery of the flexible pipe body is provided to prevent the deflection of the flexible pipe body.