An apparatus is provided for transmitting data in an airbag activation system mounted on a vehicle. The apparatus comprises a communication channel, a plurality of sensor units mutually communicably connected via the communication channel, and a processing unit. Each sensor unit senses a state of the vehicle, produces data indicative of the sensed state of the vehicle into a set of digital data including actually sensed data of a predetermined bit length, the actually sensed data corresponding to the sensed state of the vehicle, and outputs the set of digital data to the communication channel at predetermined transmission intervals. The bit length of the actually sensed data is changed depending on the sensed state of the vehicle. The set of digital data is outputted successively from the sensor modules. The processing unit successively receives the digital data from each sensor unit via the communication channel.