In a vehicular system (1) configured such that data can be transmitted and received via a data bus (4) between an engine electronic control unit (2) for controlling an engine and electronic units (3-1 to 3-n) including a data storing function, the most recent content of accumulated data stored in the engine electronic control unit (2) is stored as backup data in the electronic units (3-1 to 3-n) via the data bus (4) at an appropriate timing, and when the engine electronic control unit (2) is replaced, the necessary backup data is transferred via the data bus (4) from any of the electronic units to an engine electronic control unit (2) after replacement.