Updating of a program is realized even with a small-sized temporary storage area. A program update system distributes an update program from an update program distributing unit to a vehicle control device. The update program distributing unit encrypts an update program by a predetermined unit of encryption, and transmits the update program, which has been encrypted, by a predetermined unit of transfer to the vehicle control device to be updated, the vehicle control device decrypts the received update program by the unit of encryption, and stores the update program, which has been decrypted, by a predetermined unit of writing in a program storage unit, and the unit of writing is an integral multiple of the unit of encryption.