1. An image processing system comprising: a component marked with a feature whose position and attitude relative to an imaging device can be specified by being imaged by the imaging device; and an image processing apparatus connected to the imaging device, the image processing apparatus comprising a computer processor configured to perform at least: acquiring a captured image captured by the imaging device; detecting the feature from the captured image; acquiring a position/attitude reference based on the detected feature, the reference being used as a reference for indicating a position and attitude of the detected feature in a space imaged by the imaging device; acquiring, based on a first captured image from which a plurality of features is detected, relative relation information indicating a relative relation in position and attitude among a plurality of position/attitude references acquired for each of the plurality of features; storing the relative relation information; and estimating the position/attitude references of one or more features which is detected from the first captured image and is not detected from a second captured image which is a new captured image, based on the relative relation information and one of the plurality of position/attitude references of another feature which is detected from the first and second captured image.