1. An image pickup system configured to perform an estimation for an unknown spectral characteristic related to a subject or an illumination light on the basis of an image signal from a color image sensor system, the image pickup system comprising: a control unit for performing a control to change a predetermined spectral characteristic in the color image sensor system in a time series manner and cause a plurality of image pickup operations; a verification unit for verifying a reliability related to the plurality of image signals obtained from the plurality of image pickup operations; a correction unit for performing a correction processing on the plurality of image signals on the basis of the reliability; and an estimation unit for performing the estimation for an unknown spectral characteristic related to the subject or the illumination light on the basis of the plurality of image signals on which the correction processing is performed.