A method and apparatus for estimating photmultiplier sensitivity change is provided, which is capable of properly and easily estimating the sensitivity change in a photomultiplier used in a radiation image reading unit constructed to read out a radiation image using the photomultiplier. The method and apparatus obtain exposure conditions used when the radiation image was recorded on the storage phosphor sheet by the exposure condition obtaining means, then calculate an estimated amount of radiation irradiated on the storage phosphor sheet by the sensitivity change information obtaining means based on the exposure conditions obtained by the exposure condition obtaining means, cumulatively add the estimated amount of radiation for each radiation image recorded on the storage phosphor sheet, and obtain the cumulative value of the estimated amounts as the information that indicates the sensitivity change in the photomultiplier.