A system comprising: a first information acquisition means configured to acquire first information indicating a detection value of a first physical quantity in a monitored area; a second information acquisition means configured to acquire second information indicating a detection value of a second physical quantity different from the first physical quantity, in the monitored area; a determination means configured to determine whether an abnormality has occurred in the monitored area, based on the detection value of the first physical quantity indicated by the first information, the detection value of the second physical quantity indicated by the second information, a first coefficient corresponding to the detection value of the first physical quantity and the detection value of the second physical quantity, and a first threshold; a first detection means configured to detect the concentration of the first detection target substance in the monitored area as the first physical quantity; a second detection means configured to detect the concentration of the second detection target substance different from the first detection target substance, in the monitored area as the second physical quantity; and a holding means configured to hold, as a hold value, a past detection value by the first detection means or the second detection means or a predetermined value as a detection value of at least one of the first detection means or the second detection means, only in a predetermined case, wherein the first information acquisition means acquires the detection value of the first detection means as the first information, the second information acquisition means acquires the detection value of the second detection means as the second information, and only in the predetermined case, the determination means considers the hold value held by the holding means as the detection value of the first physical quantity indicated by the first information or the detection value of the second physical quantity indicated by the second information to determine whether the abnormality has occurred, if the first detection means or the second detection means detects lower concentration than a predetermined concentration after detecting higher concentration than the predetermined concentration, the holding means determines to be in the predetermined case and holds the hold value based on the predetermined concentration. a first information acquisition means configured to acquire first information indicating a detection value of a first physical quantity in a monitored area; a second information acquisition means configured to acquire second information indicating a detection value of a second physical quantity different from the first physical quantity, in the monitored area; a determination means configured to determine whether an abnormality has occurred in the monitored area, based on the detection value of the first physical quantity indicated by the first information, the detection value of the second physical quantity indicated by the second information, a first coefficient corresponding to the detection value of the first physical quantity and the detection value of the second physical quantity, and a first threshold; a first detection means configured to detect the concentration of the first detection target substance in the monitored area as the first physical quantity; a second detection means configured to detect the concentration of the second detection target substance different from the first detection target substance, in the monitored area as the second physical quantity; and a holding means configured to hold, as a hold value, a past detection value by the first detection means or the second detection means or a predetermined value as a detection value of at least one of the first detection means or the second detection means, only in a predetermined case, wherein the first information acquisition means acquires the detection value of the first detection means as the first information, the second information acquisition means acquires the detection value of the second detection means as the second information, and only in the predetermined case, the determination means considers the hold value held by the holding means as the detection value of the first physical quantity indicated by the first information or the detection value of the second physical quantity indicated by the second information to determine whether the abnormality has occurred, if the first detection means or the second detection means detects lower concentration than a predetermined concentration after detecting higher concentration than the predetermined concentration, the holding means determines to be in the predetermined case and holds the hold value based on the predetermined concentration.