Provided is a photochemical reaction device wherein two partitions formed from an optically transparent material are arranged apart from each other between a light source and a reaction liquid, and an optically transparent fluid introduction/discharge means for introducing an optically transparent fluid between the partitions and discharging the fluid and a state change detection means for detecting a change in the state of the optically transparent fluid at the discharge side of the optically transparent fluid introduction/discharge means are provided. Also provided are a photochemical reaction method that uses the photochemical reaction device and a lactam production method that uses the photochemical reaction method. The present invention prevents decreases in the performance of the light source even when the optically transparent material in the photochemical reaction device is damaged, and makes it possible to reliably prevent ignition even if the reaction liquid is a flammable liquid.
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