To make drainage possible at a shallow pit such as the bottom of an excavated pit by a method wherein a low level drainage pump having a suctiion port at its bottom is surrounded with a screen and a water level detecting device is provided, and the pump is automatically stopped when the water level reach the minimum allowable level.

A suction port 3c is provided at the bottom of a drainage pump body 3a while legs 3b are formed short to the utmost, and the drainage pump 3 is surrounded with a dustproof screen 7 and thereby spring water at a shallow pit can be drained effectively. A water level detecting device for detecting a startinng water level, a stopping water level and a warning water level for the drainage pump 3 is provided, and the drainage pump 3 is automatically stopped when the water level comes down to a point where the drainage becomes impossible with air taken in, and automatically started agains when the watrer level rises. Therefore, drainage at a shallow pit such as the bottom of an excavated pit can be executed without providing a pool for making the drainage pump operable. Thereby, works for constructing a pit can be simplified while improving efficiency in execution of work.