This paper describes the stabilizing mechanism of a slurry trench excavation in soft clay under normal consolidation. Diaphragm wall construction, using the slurry trench method, is increasingly being applied to soft clay. To ensure the reliability of this method, it is important to clarify the stabilizing mechanism of slurry trench excavation. With regard to soft clay, Dibiagio et al. (1972), Aas (1976), Tamano et al. (1984), and Kanatani et al. (1984) discussed stress conditions in the trench wall. In this research it was recognized that the mechanism of trench wall stability in soft clay was to be studied. The experimental trench used for our study was 1 m wide, 9.5 m long, and 21 m deep. The clay layer comprises normally consolidated clay with an undrained shear strength of 29-73 kPa. Mechanical behavior measurements during experimental trench excavation are shown. Based on these measurements, the mechanism of trench wall stability is discussed; lateral inward displacement of the trench wall (up to 20 mm) caused the lateral pressure (earth and water pressures combined) acting on the trench wall to decrease below its original level at rest (before the trench excavation) and then to reach a balance with the slurry pressure, thereby maintaining trench wall stability. Consequently, normally consolidated soft clay yields only a slight trench displacement which achieves a balance between slurry and lateral pressures, thus permitting use of the diaphragm wall employing the slurry trench method.