A packing box enables easy release a slit lock in which slits in an insertion piece of an openable lid engage side flaps connected to openable lid side edge portions of lid side members, even if the interior of a box body of the packing box is tightly filled with a product. The top face lid of the packing box includes a part that can be torn along a tear inducing line extending from the flap opposing edge of the top face lid to the front end edge of the top face lid. This part is held and torn along the tear inducing line by a human hand, and is raised up with respect to the top face lid to form the slit lock release gripping piece that can be used for releasing engagement created by the slit lock engaging structure, so that the engagement by the slit lock engaging structure, which is the engagement between the insertion piece and the side flap in the slit on the side in the lateral direction of the insertion piece where the slit lock release gripping piece is formed, can be easily released even if the interior of the box body of the packing box is tightly filled.