A tool box includes a case body and two side plates. The case body includes base and cover parts. The lateral sides of the base and cover parts have peripheral flanges that extend along the peripheries of the lateral sides to confine cavity halves. The peripheral flanges of the base part have opposite sides formed with first engagement portions. Each of the side plates has a lower portion formed with a second engagement portion that engages releasably a corresponding one of the first engagement portions. The side plates further have outer surfaces adapted to be provided with marks. The lower and upper portions of the side plates are retained in respective cavity halves of the base and cover parts. Stop members project from the peripheral flanges adjacent to the end faces outwardly of the cavities.