A diffractive optical device comprising at least one diffractive optical element (12, 17), wherein the at least one diffractive optical element (12, 17) generates light having a first order and light having a second order from a laser beam input to the at least one diffractive optical element (12, 17), and superimposes the light having the first order and the light having the second order on each other on an optical axis of the laser beam to cause interference between the light having the first order and the light having the second order, the light having the first order and the light having the second order having diffraction orders different from each other, the at least one diffractive optical element (12, 17) includes a first phase pattern and a second phase pattern, the first phase pattern converts the laser beam into a line beam, the second phase pattern diffracts the laser beam in a short axis direction of the line beam to generate the light having the first order and the light having the second order, a first focal plane of the light having the first order is located at a position different from a second focal plane of the light having the second order on the optical axis the second phase pattern includes a central phase pattern and peripheral phase patterns disposed on both sides relative to the central phase pattern in the short axis direction, the central phase pattern provides a first optical phase to the laser beam, each of the peripheral phase patterns provides a second optical phase different from the first optical phase to the laser beam, and a difference between the first optical phase and the second optical phase is π. the at least one diffractive optical element (12, 17) generates light having a first order and light having a second order from a laser beam input to the at least one diffractive optical element (12, 17), and superimposes the light having the first order and the light having the second order on each other on an optical axis of the laser beam to cause interference between the light having the first order and the light having the second order, the light having the first order and the light having the second order having diffraction orders different from each other, the at least one diffractive optical element (12, 17) includes a first phase pattern and a second phase pattern, the first phase pattern converts the laser beam into a line beam, the second phase pattern diffracts the laser beam in a short axis direction of the line beam to generate the light having the first order and the light having the second order, a first focal plane of the light having the first order is located at a position different from a second focal plane of the light having the second order on the optical axis the second phase pattern includes a central phase pattern and peripheral phase patterns disposed on both sides relative to the central phase pattern in the short axis direction, the central phase pattern provides a first optical phase to the laser beam, each of the peripheral phase patterns provides a second optical phase different from the first optical phase to the laser beam, and a difference between the first optical phase and the second optical phase is π.