An in-vehicle network system includes a first control unit, a second control unit, and a network connecting the first control unit to the second control unit. The first control unit performs a process of transmitting data, which is target of high-speed processing, having a first data length obtained by dividing transmission data by the first data length or obtained by adding predetermined data to the transmission data as high-speed processing. The second control unit performs a process of receiving reception data at a first processing speed that is a low speed when the second control unit determines that a data length of the reception data is not the first data length, and performs a process of receiving the reception data at a second processing speed higher than the first processing speed when the second control unit determines that the data length of the reception data is the first data length.