The present invention provides a vehicle-mounted control device, a program writing device, a program generating device, and a program, which are capable of quickly and easily carrying out reprogramming. An ECU 300 is provided with: a microcomputer 301, an SRAM 302, a flash memory 303, and a communication device 305. The flash memory 303 is configured from a plurality of blocks and stores older versions of the program. The communication device 305 receives (S250) a frame including block data, the type of the block data, and an address of a block in which the block data is to be written. The microcomputer 301 restores (S260) a new program from the block data in the SRAM 302 according to the type of the block data (S255, S260) and writes (S265, S270) one piece of the restored new program in a block corresponding to the address.