A first-type data transfer frame for uncompressed transfer of a control program and control data including reference data, and a second-type data transfer frame for compressed transfer have the same fixed size, and each transfer frame includes compression identification information indicating uncompressed or compressed data. When an on-board vehicle control unit receives a transfer frame with uncompressed control data, the control unit stores the uncompressed control data of the frame in a nonvolatile memory. When the transfer frame has compressed control data, the control unit writes the compressed control data of the frame into RAM, and decompresses control data in the RAM. The control unit sequentially accumulates decompressed control data in the RAM, and when amount of the accumulated control data reaches a setting amount, the control unit halts the decompressing, and transfers the decompressed control data from the RAM into the nonvolatile memory, and restarts decompressing for the next received data transfer frame from the halt point.