To provide an inexpensive drain flowmeter.

The drain flowmeter includes: a primary pressure sensor 12 and a secondary pressure sensor 13 attached to an inlet 4 side and an outlet 6 side of a free float type drain trap 19, respectively; a case 20 projectingly fixed to the outside from a valve chamber 3; a rotary plate 22 rotationally attached around a center shaft 21 of the case 20; a float arm 23 whose one end is fixed to the rotary plate 22 and whose other end abuts on the upper end of a float 9 and rotating by moving up and down the float 9 so as to rotate the rotary plate 22; a movable shaft 25 linked to the rotary plate 22 and displaced in a shaft center direction of the case 20 accompanied with the rotation of the rotary plate 22; a permanent magnet 26 built inside the movable shaft 25; a magnetic sensor 27 for detecting a position of the float 9 on the basis of a distance change with the permanent magnet 26 corresponding to the displacement of the permanent magnet 26; and flow rate calculation means 28 for calculating an opening degree of a valve port 7 from the position of the float 9 and for calculating a drain flow amount flowing down the valve port 7 from the primary and secondary pressures.