A lid forming device of a cardboard box using a first flap with a slit and a second flap to be extended after being bent into a mountain shape is disclosed. The lid forming device includes first and second folding mechanisms. The fist folding mechanism folds the first flap in a direction of closing the opening. The second folding mechanism bends the second flap into a mountain shape and inserts part of the second flap into the slit of the first flap while extending the second flap. The second folding mechanism includes first and second retaining units. The first retaining unit retains a first face of the second flap located between a free end of the second flap and a bend line set beforehand in the second flap. The second retaining unit retains a second face of the second flap located between an anchored end and the bend line.