A configuration is provided so that information of detection of vehicles, pedestrians, etc. provided from a roadside device can be used by an automobile that cannot communicate with the roadside device. A warning device is configured to output a warning for an automobile traveling in a predetermined lane at an intersection where a roadside device is installed, the roadside device being configured to transmit information of detection of vehicles, pedestrians, etc., the warning device including a receiving unit configured to receive information of detection of vehicles, pedestrians, etc. transmitted from the roadside device and signal information of a signal lamp for the predetermined lane, and a warning unit configured to output a warning based on the information of detection of vehicles, pedestrians, etc. and the signal information received by the receiving unit.