A plastic box formed of plastic that closes an opening by bending inner Flaps, arranged on one pair of opposite two sides at a periphery of the opening, towards the inside, and folding an outer flap arranged on one side of the other pair of opposite two sides, while inserting and latching an insertion flap piece into the inside of the opening. The plastic box includes a lock mechanism that functions by engaging respective base end parts of the inner flap with notches formed on respective end parts of a bend line bent from the outer flap of the insertion flap piece. In addition, the plastic box includes a finger-receiving brim piece protruding from the bend line towards the outside at a position shifted from the center of the bend line towards the side edge.