Provided are an on-board update device, an on-board update system, an update process method, and an update process program. The on-board update device updates a program or data stored in a storage unit of an on-board device, the on-board update device includes a communication unit performing communication via a communication line connected to the on-board device, a storage unit stores history information regarding the timing of transmission and reception of data that is to be transmitted and received via the communication line, an estimation processing unit estimates an unused time period in which no data is transmitted or received via the communication line, based on the history information stored in the storage unit, and an update processing unit performs processing for transmitting, via the communication unit data for updating the on-board device to the on-board device in the unused time period estimated by the estimation processing unit.