An image forming apparatus comprising: a plurality of paper feeding cassettes operable to be loaded in a detachable manner in the inside of an apparatus main body; a paper sheet detection sensor operable to detect a paper sheet stored in said paper feeding cassettes; a sensing sensor (109a~109c) operable to sense a paper sheet conveyed along a carrying passage (110,111) from said paper feeding cassette; a paper feeding changeover determining part operable to determine whether changeover of paper feeding is possible or not, on the basis of the operation state of the apparatus, a change in the result of detection by said paper sheet detection sensor, and the result of sensing by said sensing sensor (109a, 109b, 109c); and a system control part operable to perform processing to control changeover of paper feeding on the basis of the result of determination by said paper feeding changeover determining part, wherein said plurality of paper feeding cassettes are disposed in the inside of the apparatus main body, being vertically stacked, and each is provided with a common carrying passage that is also used for said paper sheet fed from a lower stage, characterized in that said paper feeding changeover determining part performs processing to determine that, when, in the course of printing, the conveyance of said paper sheet is not sensed by said sensing sensor (109a, 109b, 109c), if the result of detection by said paper sheet detection sensor is changed from paper sheet presence to paper sheet absence, said paper feeding cassette have been drawn out from the apparatus main body, and performs processing to notify said system control part that changeover of the paper feeding cassette to another one is impossible. a plurality of paper feeding cassettes operable to be loaded in a detachable manner in the inside of an apparatus main body; a paper sheet detection sensor operable to detect a paper sheet stored in said paper feeding cassettes; a sensing sensor (109a~109c) operable to sense a paper sheet conveyed along a carrying passage (110,111) from said paper feeding cassette; a paper feeding changeover determining part operable to determine whether changeover of paper feeding is possible or not, on the basis of the operation state of the apparatus, a change in the result of detection by said paper sheet detection sensor, and the result of sensing by said sensing sensor (109a, 109b, 109c); and a system control part operable to perform processing to control changeover of paper feeding on the basis of the result of determination by said paper feeding changeover determining part, wherein said plurality of paper feeding cassettes are disposed in the inside of the apparatus main body, being vertically stacked, and each is provided with a common carrying passage that is also used for said paper sheet fed from a lower stage, characterized in that said paper feeding changeover determining part performs processing to determine that, when, in the course of printing, the conveyance of said paper sheet is not sensed by said sensing sensor (109a, 109b, 109c), if the result of detection by said paper sheet detection sensor is changed from paper sheet presence to paper sheet absence, said paper feeding cassette have been drawn out from the apparatus main body, and performs processing to notify said system control part that changeover of the paper feeding cassette to another one is impossible.