A drainage valve unit for use with a casing for collecting a drain includes a valve body adapated to be mounted on the casing and having a fluid passage and a chamber defined therein and communicating with each other, a valve member disposed in the fluid passage and movable axially of the fluid passage for opening and closing the fluid passage, and a pusher body disposed in the chamber and movable axially of the chamber, the pusher body being displaceable from outside of the valve body for causing the valve body to open the fluid passage to discharge the drain from the casing through the fluid passage. The valve member can reliably be moved to open the fluid passage for discharging the drain by pushing the pusher body irrespective of the pressure within the casing. When the fluid passage is opened, the drain can quickly be discharged without falling on the operator's hand and fingers.