Water (or moisture) content of soil-mix in a plug tray with limited-volume can easily be changed by evapotranspiration from a plug sheet and watering. Thus, timing and the amount of watering are the most important skills in plug seedling production. An automatic watering system for controlling water content of soil-mix in a plug tray was developed. A tomato plug sheet consisting of plug seedlings, soil mix in a greenhouse was watered by the watering system. The weight of the plug sheet was measured continuously with an electronic balance. When the weight of the plug-sheet decreased to a lower-limit setpoint by evapotranspiration, a solenoid switch of the watering system with overhead mist spray nozzles was turned on, and the plug sheet was watered. When the weight of the plug-sheet increased to a higher-limit setpoint by watering, the solenoid switch of the watering system was turned off. Watering was conducted only during the daytime. In this way, the weight of the plug sheet was controlled accurately in a range between the lower- and higher-limit setpoints during the culture period.