1. An image processing apparatus comprising: an image input unit configured to input an image of a work; a work state estimation unit configured to estimate a state of the work from the image which has been input by the image input unit; an occurrence probability storage unit configured to store an occurrence probability which is previously associated with each state of the work; and a reliability calculation unit configured to calculate reliability for the estimated state of the work, based on the state of the work which has been estimated by the work state estimation unit and the occurrence probability which corresponds to the state of the work.