Provided is a photocatalyst device that can be implemented using a simple configuration, in which a photocatalyst filter can be cleaned and the performance of the filter restored during maintenance without removal of the filter from the position at which the filter is mounted (while the filter remains mounted). The photocatalyst device comprises a substantially flat plate-shaped photocatalyst filter module (20) including a photocatalyst filter (30) mounted in a substantially vertical direction a cleaning-water supply tube (31) having a slit, the cleaning-water supply tube being secured to the module (20) above the photocatalyst filter (30) and a cleaning-water storage unit (32) having a discharge outlet (21a) and a lid member, the cleaning-water storage unit being provided to the module (20) below the photocatalyst filter (30). When cleaning water (W) is supplied from the exterior to the cleaning-water supply tube (31), the cleaning water (W) flows from the slit along the photocatalyst filter (30) and is temporarily stored in the cleaning-water storage unit (32). When the cleaning water (W) reaches a predetermined amount, the lid member opens, and the cleaning water is discharged from the discharge outlet (21a).
