A laser treatment device performing treatment by irradiating a target object having a plate surface with laser light, including: a light-transmitting region transmitting laser light emitted onto the target object; a rectifier that has a rectifier surface separated from the target object and extending along the plate surface of the target object and outward from the end of the light-transmitting region; a gas supply unit that feeds a gas to a gap between one side of the rectifier surface and the light-transmitting region, in a position separated from the light-transmitting region; and a gas exhaust unit that exhausts, on the other side that is on the other side of the light-transmitting region from the one side, the gas present in a gap between the rectifier surface and the target object from the gap, in a position separated from the light-transmitting region, thereby generating a stable local gas atmosphere.