PROBLEM TO BE SOLVED: To provide a decontamination method and a decontamination device capable of easily determining condensation, of securely decontaminating the inside of a room in a drying method without using a vaporization type device, and of requiring no expert operator.SOLUTION: In a room 1 are disposed a mist spraying unit 2 for spraying mist of hydrogen peroxide containing water and an agitation fan 3 for spreading the mist in the room 1. Based on a temperature &thetas and a hydrogen peroxide concentration C in the room 1 displayed on a power control panel 11, a relative humidity &phgrat which dew condensation occurs is obtained from a line diagram indicating a saturated vapor pressure in an air which is a two-component mixture of the hydrogen peroxide and the water, printed on a document 16. When it is determined that a relative humidity &phgr in the room 1 which is displayed on the power control panel 11 as a humidity setting value or control relative humidity &phgr, which is a relative humidity &phgror saturated relative humidity, or a value displayed by being reduced only by a predetermined value from the relative humidity &phgr, is equal to or higher than the control relative humidity &phgr, a forced stop switch 17 is pressed, a closing command is output to a shutoff valve 6, and then decontamination is stopped and a starting command is output to a detoxification device 10.
