We present a novel method that robustly estimates the reﬂectance, even in an environment with dynamically changing light. To control the appearance of an object by using a projector–camera system, an appropriate estimate of the object’s reﬂectance is vital to the creation of an appropriate projection image. Most conventional estimation methods assume static light conditions; however, in practice, the appearance is affected by both the reﬂectance and environmental light. In an environment with dynamically changing light, conventional reﬂectance estimation methods require calibration every time the conditions change. In contrast, our method requires no additional calibration because it simultaneously estimates both the reﬂectance and environmental light. Our method is based on the concept of creating two different light conditions by switching the projection at a rate higher than that perceived by the human eye and captures the images of a target object separately under each condition. The reﬂectance and environmental light are then simultaneously estimated by using the pair of images acquired under these two conditions. We implemented a projector–camera system that switches the projection on and off at 120 Hz. Experiments conﬁrm the robustness of our method when changing the environmental light. Further, our method can robustly estimate the reﬂectance under practical indoor lighting conditions.