A control device that controls a monitoring device having an autonomous travel function, the control device comprising: a detecting section that detects a change in a target of monitoring of the monitoring device; and an operation mode deciding section that decides operation modes of the monitoring device according to a detail of the change in the target of monitoring detected by the detecting section, wherein the monitoring device has a work instrument for executing a second work different from a first work selected from a group consisting of a monitoring work, a security work and a guard work for the target of monitoring, the operation mode deciding section has: an alert level deciding section that decides an alert level indicating a level of threat to the target of monitoring, according to the detail of the change in the target of monitoring detected by the detecting section; a first operation mode deciding section that decides a first operation mode indicating a detail of an operation about the first work, according to the alert level decided by the alert level deciding section; and a second operation mode deciding section that decides a second operation mode indicating a detail of an operation about the second work, according to the alert level decided by the alert level deciding section, wherein the control device controls the monitoring device such that the first operation mode and the second operation mode corresponding to the decided alert level are performed simultaneously. a detecting section that detects a change in a target of monitoring of the monitoring device; and an operation mode deciding section that decides operation modes of the monitoring device according to a detail of the change in the target of monitoring detected by the detecting section, wherein the monitoring device has a work instrument for executing a second work different from a first work selected from a group consisting of a monitoring work, a security work and a guard work for the target of monitoring, the operation mode deciding section has: an alert level deciding section that decides an alert level indicating a level of threat to the target of monitoring, according to the detail of the change in the target of monitoring detected by the detecting section; a first operation mode deciding section that decides a first operation mode indicating a detail of an operation about the first work, according to the alert level decided by the alert level deciding section; and a second operation mode deciding section that decides a second operation mode indicating a detail of an operation about the second work, according to the alert level decided by the alert level deciding section, an alert level deciding section that decides an alert level indicating a level of threat to the target of monitoring, according to the detail of the change in the target of monitoring detected by the detecting section; a first operation mode deciding section that decides a first operation mode indicating a detail of an operation about the first work, according to the alert level decided by the alert level deciding section; and a second operation mode deciding section that decides a second operation mode indicating a detail of an operation about the second work, according to the alert level decided by the alert level deciding section, wherein the control device controls the monitoring device such that the first operation mode and the second operation mode corresponding to the decided alert level are performed simultaneously.