The light-addressable potentiometric sensor (LAPS) is a chemical sensor based on the field effect in the electrolyte - insulator - semiconductor (EIS) structure. In the conventional LAPS, the ion concentration in the analyte solution is determined by measuring the amplitude of the photocurrent induced by illumination of a modulated light. In this study, we developed the phase-mode LAPS, in which the phase of the alternating photocurrent is measured instead of its amplitude. The phase - voltage curve shifted along the voltage axis in response to the variation of ion concentration, in analogy to the shift of the current - voltage curve in the conventional LAPS. The primary advantage of the phase-mode LAPS is that it is much less sensitive to the fluctuation of the light intensity. It is also demonstrated that chemical imaging based on the phase-mode LAPS is almost free from artifacts related to defects in the semiconductor substrate.