This paper proposes a method for estimating the omnidirectional distribution of the scene illuminant spectrum from images taken by a camera aimed at a mirrored ball. First, we introduce measuring systems using a RGB color camera and a mirrored ball placed at an arbitrary location in a natural scene for acquiring ambient light from any direction. Second, in order to estimate the direction from which light comes to the ball, we calibrate the measuring systems and determine a mapping between the image coordinates of the spherical ball and the directions of light rays in the world. An omnidirectional image representing the spatial distribution of ambient light sources is created in a polar coordinate system. Third, we present an algorithm for recovering the illuminant spectral power distribution from the image data at each pixel and the reflectance data of the mirrored ball. Finally, we execute an experiment in a natural environment with multiple light sources for examining the feasibility of the proposed method.