We present a novel method for robustly estimating reflectance even in a dynamically changing light environment. To control the appearance of an object by using a projector-camera system, appropriate reflectance estimation is vital to the creation of an appropriate projection image. Most conventional estimation methods have assumed a static light condition although, in practice, the appearance is affected not only by the reflectance but also by the environmental light. In a dynamically changing light environment, conventional methods must be capable of applying calibrations every time the conditions change. Our method is based on a concept of creating two different light conditions by switching the projection at a rate that is higher than what the human eye can perceive, capturing images of a target object separately under each condition. Our method simultaneously estimates the reflectance and environmental light by using a pair of images acquired under two conditions. We implemented a projector-camera system that switches the projection on and off at 120 Hz. We conducted two experiments to confirm the robustness when changing the brightness and the color saturation of the environmental light, respectively. The results indicated our method is capable of robustly estimating the reflectance under practical indoor lighting conditions.