The transmission server according to the present invention includes a processor, buffer memory, an I/O bus, and a transmission unit. Furthermore, the transmission unit includes a transmitting unit for controlling the generation and transmission of a packet, buffer memory for holding transmission data provided by the processor, and an NIC for connection to a network. The processor manages receivers, and transfers data in the buffer memory to be transmitted to the transmission unit connected to the I/O bus. The transferred data is stored in the buffer memory of the transmission unit. The transmission unit generates a packet for each receiver from the data in the buffer memory according to the receiver information, and transmits the data to a network through the NIC.