To keep the same level between pressure drops of filters in a time for the replacement as much as possible by making the time for replacing a dust collection filter variable by using the level of a used airflow.

An air purifier determines whether the times for replacement comes for an ultraviolet lamp 12, a photocatalytic filter 13, and the dust collection filter 11 in this order, respectively (S3, S12, S21). If the time for the replacement comes, a corresponding replacement sign 17 flashes (S4, S13, S23), and the device is forced to stop (S29). If one of the items is within a time to display the replacement sign 17 (S5, S20, S22: YES), the air purifier determines whether the time for the replacement of the other items are within an adjustment time (S6, S8, S14, S16, S24, S26). If it is true, the corresponding replacement sign 17 is turned on (S7, S9, S11, S15, S17, S19, S25, S27, S30). A time for replacing the dust collection filter 11 is calculated by converting an operation time in the airflow in use into an operation time in a low airflow (S38).