A communication system including a plurality of devices capable of wireless multi-hop communication, the communication system comprising: a storage unit configured to store location information of each of the plurality of devices, site configuration information including location information and size information of an obstacle present at a site of the communication system where the plurality of devices are provided, and communication quality information between devices of the plurality of devices; a determination unit configured to determine whether a repeater is required to be added for each of the plurality of devices, based on the communication quality information; and a decision unit (408, 409) configured to calculate an expected value related to attenuation of received power between a first device and a second device caused by the obstacle present at the site and configured to decide a location at which the repeater is to be arranged, based on location information of the first device for which it was determined that a repeater is required to be added by the determination unit, and based on location information of the second device different from the first device and communicating with the first device through the repeater, and based on the communication quality information on communication quality between the first device and the second device, and based on the site configuration information, wherein the decision unit (408, 409) is configured to decide, when the obstacle exists between the first device and the second device, and the expected value related to the attenuation of the received power caused by the obstacle is equal to or greater than a threshold value in the received power between the first device and the second device, the location of the repeater to be a location within a communication range where a communication range of the first device and a communication range of the second device overlap and at which neither a straight line connecting the first device and the repeater nor a straight line connecting the repeater and the second device cross the obstacle, and is configured to decide, when the obstacle exists between the first device and the second device, and the expected value related to the attenuation of the received power caused by the obstacle is less than a specified value in the received power between the first device and the second device, the decision unit (408, 409) is configured to decide the location of the repeater to be a location within a communication range where a communication range of the first device and a communication range of the second device overlap and at which one or both of a straight line connecting the first device and the repeater and a straight line connecting the repeater and the second device cross the obstacle. a storage unit configured to store location information of each of the plurality of devices, site configuration information including location information and size information of an obstacle present at a site of the communication system where the plurality of devices are provided, and communication quality information between devices of the plurality of devices; a determination unit configured to determine whether a repeater is required to be added for each of the plurality of devices, based on the communication quality information; and a decision unit (408, 409) configured to calculate an expected value related to attenuation of received power between a first device and a second device caused by the obstacle present at the site and configured to decide a location at which the repeater is to be arranged, based on location information of the first device for which it was determined that a repeater is required to be added by the determination unit, and based on location information of the second device different from the first device and communicating with the first device through the repeater, and based on the communication quality information on communication quality between the first device and the second device, and based on the site configuration information, wherein the decision unit (408, 409) is configured to decide, when the obstacle exists between the first device and the second device, and the expected value related to the attenuation of the received power caused by the obstacle is equal to or greater than a threshold value in the received power between the first device and the second device, the location of the repeater to be a location within a communication range where a communication range of the first device and a communication range of the second device overlap and at which neither a straight line connecting the first device and the repeater nor a straight line connecting the repeater and the second device cross the obstacle, and is configured to decide, when the obstacle exists between the first device and the second device, and the expected value related to the attenuation of the received power caused by the obstacle is less than a specified value in the received power between the first device and the second device, the decision unit (408, 409) is configured to decide the location of the repeater to be a location within a communication range where a communication range of the first device and a communication range of the second device overlap and at which one or both of a straight line connecting the first device and the repeater and a straight line connecting the repeater and the second device cross the obstacle.