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 a bactericidal agent containing water and an agitation fan 3 for spreading the mist in the room 1. In the room 1 is provided a power control panel 11 which obtains a parameter aand a parameter bbased on an indoor temperature &thetas obtains a line diagram indicating a saturated vapor pressure in an air which is a two-component mixture of the bactericidal agent and the water by a primary approximate expression &phgr=aC+b obtains a relative humidity &phgrat which dew condensation is caused by a concentration C of the bactericidal agent on the basis of the primary approximate expression obtains a control relative humidity &phgron the basis of the relative humidity outputs an opening command to start decontamination to a shutoff valve 6, when a relative humidity &phgr measured by a hygrometer 8 is lower than the control relative humidity &phgr and outputs a closing command to stop decontamination to the shutoff valve 6, when the relative humidity &phgr is equal to or higher than the control relative humidity &phgr, and also outputs a starting command to a detoxification device 10.
