To provide an on-vehicle device capable of minimizing an increase in the size of the device by preventing the operation of the device from being interrupted for a long time even if an engine is started during the operation when a hard disk device having a write cache memory is used.

This on-vehicle device 100 includes a hard disk device 30 allowing the designation of use/nonuse of the write cache memory 34, a product microcomputer section 10, a power supply circuit 60 for supplying an operation power to the hard disk device 30, a power supply circuit 62 to which a booster circuit 64 is connected and which supplies an operation power to the product microcomputer section 10, and a CPU 12 serving as a CM control section which instructs the unuse of the write cache memory 34 to the hard disk device 30 before the start of the engine and also instructs the use of the write cache memory 34 to the hard disk device 30 after the start of the engine.