A three-dimensional printing device includes an optical device and a controller that controls the optical device. The controller includes a dividing section that divides an area of a vat that is irradiated with light of a projector into a plurality of blocks, a counting section that counts a number of times of irradiation with light of the projector, a first determination section that determines whether or not the number of times counted by the counting section has reached a predetermined threshold value, a light output section that changes when it is determined by the first determination section that the number of times has reached the predetermined threshold value from a first energy to a second energy lower than the first energy an energy of light to be output from the projector to the photo-curable resin of the block for which determination has been made.