The invention provides a packaging box that maintains a state in which side wall bodies hold extension portions therebetween. A stacking tray (1) (packaging box) designed in such a manner that a plurality thereof can be stacked vertically includes: a bottom wall body (10) that has an engagement recessed portion formed on a surface thereof in a recessed manner; end wall bodies (11) disposed uprightat a rim portion of the bottom wall body (10); and a side wall bodies (12) disposed upright at a rim portion of the bottom wall body (10), adjacent to the end wall bodies (11). Each of the end wall bodies (11) has support walls (35) that are provided so as to protrude toward the inside and extension portions that are provided so as to extend toward the side wall bodies (12). Each of the side wallbodies (12) has an outer plate portion (20) that is provided so as to extend upward from the corresponding rim portion of the bottom wall body (10), an inner plate portion (22) that is folded downward so as to cover an upper part of each of the extension portions and sandwich the extension portions with the outer plate portion (20), and an engagement projection (23) that is provided so as to be continuous with a front end portion of the inner plate portion (22) and comes into engagement with the engagement recessed portion along the bottom wall body (10).