To provide a drain flowmeter capable of easily measuring a flow rate of drain.

A drain discharge pipe 2 is connected to an outlet side of a steam trap 1. A cooling fluid supply pipe 3 and a first flowmeter 4 are mounted to the drain discharge pipe 2. A drain branch pipe 5 is provided by branching the drain discharge pipe 2, and a second flowmeter 6 is mounted to the drain branch pipe 5. A computing section is provided for computing a flow rate of drain discharged from the steam trap 1 from a differential between flow rate values detected by the second flowmeter 6 and the first flowmeter 4. A flow rate of cooling water supplied from the cooling fluid supply pipe 3 is measured by the first flowmeter 4, a flow rate of a mixed fluid of the flow rate of cooling water and the flow rate of drain discharged from the steam trap 1 is measured by the second flowmeter 6, and the flow rate of drain discharged from the steam trap 1 can be measured from a differential of these flow rates.