The aggregation number of a nonionic surfactant micelle, Triton X 100 (TX100), in aqueous solution was determined as a function of pressure by using the method of steady-state fluorescence quenching. The method of this work uses the fluorescence quenching of a probe (pyrene) by a quencher (coumarin 153), which are solubilized within a micelle. With increasing pressure, the aggregation number of TX100 takes a minimum. Namely, it decreases from 250 at atmospheric pressure down to 80 at around 100-150 MPa and then increases up to 230 at 500 MPa, the highest pressure studied. This behavior is closely related to the turnover phenomenon of critical micelle concentration (cmc) against pressure. By taking the pressure effect on the micellar concentration into account, it is demonstrated that in addition to the equilibrium between dispersed state and micellar state, there are equilibria among different-sized micelles.