Attempts were made to develop an instrument for monitoring fresh weight of leaf lettuce. First, the characteristics of a load cell, which is a component of the instrument, were investigated. Output voltage of the load cell was most stable when it was excited by a stabilized electric power source. Effects of temperature change and hysteresis after long-term loading were negligible. The readings of the weight determined by a device, equipped with a load cell, well agreed with the values of fresh weight of the plant part above the water level. This result suggests that the values of the reading be almost the same as the values of actual top fresh weight, provided that the level of water, in which the roots are submerged, is sufficiently high. Using the newly developed instrument for monitoring top fresh weight, changes of fresh weight and relative growth rate in a plant community of leaf lettuce were monitored for a 17-day period of cultivation. The fresh weight increased 23-times during the cultivation, and the relative growth rate varied in the range from 10 to 29%. These values agreed with the actual measurements taken at the same time by destructive methods. It is therefore concluded that the fresh weight of leaf lettuce plants can be accurately monitored with the instrument developed in this study.