An outside monitoring device of a mobile robot for measuring a direction of a target object using a tag, which is provided on a target object and replies a receipt signal to the mobile robot in compliance with a light signal from the mobile robot, the outside monitoring device including a plurality of light emitters each irradiating the light signal to an assigned detectable region among detectable regions established around the mobile robot are provided on the mobile robot, wherein light emitters are arranged around a reference axis orthogonal to a ground plane of the mobile robot to allow the detection of the tag within detectable regions.