A data communication system for a working machine, comprising: a data communication device configured to be attached to the working machine and configured to obtain data relating to the working machine; a mobile terminal configured to receive the data of the data communication device by wireless communication, and configured to output a working machine identification information identifying the working machine and a mobile terminal identification information identifying the mobile terminal; a first management server configured to include a user management database, the user management database registering and relating the mobile terminal of a user and the working machine sold to the user each other on the basis of the working machine identification information and the mobile terminal identification information; and a second management server configured to include a working machine information database, the working machine information database obtaining the data obtained by the data communication device via the mobile terminal and storing the obtained data, wherein the first management server outputs, to the second management server, a first authorization key for determining whether or not wireless communication between the data communication device and the mobile terminal is performed, and outputs, to the second management server, a second authorization key for determining whether or not the mobile terminal is connected to the working machine information database in a case where the mobile terminal owned by the user is related to the working machine owned by the user in the user management database, the second management server outputs the first authorization key to the mobile terminal, the first authorization key being received from the first management server, and outputs the second authorization key to the mobile terminal, the second authorization key being received from the first management server, the mobile terminal includes an authorization key output unit configured to output the first authorization key to the data communication device, and the data communication device includes a communication authorization unit configured to authenticate the first authorization key outputted from the mobile terminal with the first authorization key previously stored in the data communication device to determine validation of the first authorization key outputted from the mobile terminal, and transmits the data between the data communication device and the mobile terminal when determining validation of the first authorization key outputted from the mobile terminal. a data communication device configured to be attached to the working machine and configured to obtain data relating to the working machine; a mobile terminal configured to receive the data of the data communication device by wireless communication, and configured to output a working machine identification information identifying the working machine and a mobile terminal identification information identifying the mobile terminal; a first management server configured to include a user management database, the user management database registering and relating the mobile terminal of a user and the working machine sold to the user each other on the basis of the working machine identification information and the mobile terminal identification information; and a second management server configured to include a working machine information database, the working machine information database obtaining the data obtained by the data communication device via the mobile terminal and storing the obtained data, wherein the first management server outputs, to the second management server, a first authorization key for determining whether or not wireless communication between the data communication device and the mobile terminal is performed, and outputs, to the second management server, a second authorization key for determining whether or not the mobile terminal is connected to the working machine information database in a case where the mobile terminal owned by the user is related to the working machine owned by the user in the user management database, the second management server outputs the first authorization key to the mobile terminal, the first authorization key being received from the first management server, and outputs the second authorization key to the mobile terminal, the second authorization key being received from the first management server, the mobile terminal includes an authorization key output unit configured to output the first authorization key to the data communication device, and the data communication device includes a communication authorization unit configured to authenticate the first authorization key outputted from the mobile terminal with the first authorization key previously stored in the data communication device to determine validation of the first authorization key outputted from the mobile terminal, and transmits the data between the data communication device and the mobile terminal when determining validation of the first authorization key outputted from the mobile terminal.