To provide a paper sheet sorting device and an image forming apparatus capable of assisting a sorting operation by a user with a simple structure and simplified efforts.

There is provided a sorting device for sorting paper sheets of a predetermined size from sheet bundles in which sheets of a plurality of sizes are mixed. The sorting device comprises: a pair of guide members 10 which are positioned on both side parts in a direction orthogonal to a sheet feeding direction A, have an opening part between both side parts, can be slidably moved in the orthogonal direction and can change the opening width in the orthogonal direction; and a first sheet storage part 31 for storing a sheet guided by a pair of guide members 10 without falling from the opening part.