Provided is a photoirradiation device equipped with: a first optically transparent container for covering a light-emitting body provided with a light-emitting diode; a liquid phase section positioned to the exterior thereof and formed from a liquid having a refractive index closer to that of the first optically transparent container than that of a gas forming a gas phase section inside the first optically transparent container; and a second optically transparent container for covering the liquid phase section. Further provided are a photoreaction method using this photoirradiation device, and a method for producing lactam by using the photoreaction method. Thus, the present invention suppresses the reflection of light between a target liquid and a light-emitting diode light source, and achieves the desired photoirradiation by exhibiting high optical transparency.
