Communication system and program update method

The disclosure provides a communication system and a program update method. In a communication system according to the disclosure, when a host device updates a program stored in each of a plurality of communication terminals, first, the host device transmits a distribution preparation completion signal by broadcast. Each of the plurality of communication terminals transmits a start request signal to the host device in response to reception of the distribution preparation completion signal. The host device transmits a permission signal for giving a right for performing a data request to one communication terminal that has transmitted the start request signal earliest. The one communication terminal transmits a data request signal to the host device in response to reception of the permission signal. The host device transmits the update program data by broadcast to the plurality of communication terminals in response to reception of the data request signal.