Acoustic emissions (AE) could be used as an indicator of plant water stress as “speaking plant”. The objective of this study was to observe the AE of tomato plant and to analyze the relationship between AE and plant water consumption associated with transpiration. Three glasshouse experiments were conducted with potted tomato plants. The AE, transpiration rate and “the plant transpiration transfer coefficient (h_at)” as an indicator of plant water stress level were determined with h_at being calculated based on sunlit leaf temperature, temperature of an artificial leaf without transpiration (hereafter, referred to as “non-transpiration leaf temperature”) and air temperature. The results showed that the daily patterns of the AE varied depending on the water stress level, which was indicated by h_at. Under mild or moderate water stress (h_at ≦0) conditions, the AE increased with the decrease in the amount of soil water but decreased with the decrease in the amount of soil water under severe water stress conditions (0<h_at≦1). To analyze the hourly changes in the relationship between AE and transpiration, the concept of “change of transpiration rate (AT)” was introduced. Under mild or in the absence of water stress conditions (h_at ≦ 0), AE increased linearly with the increase of AT with a significant regression coefficient (r² = 0.85 and slope = 0.61). Then, as the water stress level increased, the r² gradually decreased, as well as the slope of the regression line between AE and AT. When the water stress level increased further, the slope continuously decreased. However, r² started to increase gradually. Thus, when the water stress increased to a critical level (0<h_at≦1), a significant inverse linear relationship between AE and AT, with r² = 0.64 and slope = –0.73 could be observed. On the basis of these results, AE tended to be differently affected by AT depending on the water stress level.