A luminaire includes a light source having a light-emitting element, and a communication device configured to transmit and/or receive a communication signal by wireless communication. The luminaire further includes a power supply configured to supply electric power to the light source and the communication device, and a housing configured to contain the communication device and the power supply. The light source is configured to be held by the housing. The housing is provided with an opening. The housing has a rectangular parallelepiped shape with electrically conductive walls to form a rectangular waveguide having a cutoff frequency. The housing is set to have a dimension in a width direction thereof such that a radio frequency of the communication signal is equal to or more than the cutoff frequency.