A data carrier system is comprised of a vehicle-side controller 12 mounted at a vehicle 11, keys 19A and 19B, and an R/W device 31 installed in an office 29. A transponder 20 is built into each of the keys 19A and 19B. In this structure, after operational data has been transmitted from the vehicle-side controller 12 and written in a nonvolatile memory of the transponder 20, when the keys 19A and 19B are inserted in the R/W device 31 by a vehicle driver, the R/W device 31 reads the operational data from the transponder 20 and gives the same to a personal computer 30. Accordingly, an inconvenience of the vehicle driver carrying an IC card for recording operational data is removed.