A transmission system comprising: a first apparatus including: a request transmitting unit configured to transmit a request packet as a request, and retransmit the request packet when a time period between the request and a response to the request exceeds a predetermined time period, and a response receiving unit configured to: receive a response packet; discard the response packet where an error has occurred; and when the response packet is a redundant response packet identical to an already received response packet, discard the redundant response packet; and a second apparatus connected to the first apparatus via a transmission path through which the request from the first apparatus to the second apparatus is transmitted and the response from the second apparatus to the first apparatus is received, the second apparatus including: a request receiving unit configured to receive the request packet transmitted by the request transmitting unit, and a response transmitting unit configured to: generate the response packet in response to the request packet received by the request receiving unit; when the request packet received by the request receiving unit includes a data write request or an interrupt request with respect to a process of the second apparatus, requiring a sequential order of request processing, generate the redundant response packet; and transmit the generated response packet and, when the redundant response packet has been generated, transmit the generated redundant response packet, to the first apparatus. a request transmitting unit configured to transmit a request packet as a request, and retransmit the request packet when a time period between the request and a response to the request exceeds a predetermined time period, and a response receiving unit configured to: receive a response packet; discard the response packet where an error has occurred; and when the response packet is a redundant response packet identical to an already received response packet, discard the redundant response packet; and receive a response packet; discard the response packet where an error has occurred; and when the response packet is a redundant response packet identical to an already received response packet, discard the redundant response packet; and a second apparatus connected to the first apparatus via a transmission path through which the request from the first apparatus to the second apparatus is transmitted and the response from the second apparatus to the first apparatus is received, the second apparatus including: a request receiving unit configured to receive the request packet transmitted by the request transmitting unit, and a response transmitting unit configured to: generate the response packet in response to the request packet received by the request receiving unit; when the request packet received by the request receiving unit includes a data write request or an interrupt request with respect to a process of the second apparatus, requiring a sequential order of request processing, generate the redundant response packet; and transmit the generated response packet and, when the redundant response packet has been generated, transmit the generated redundant response packet, to the first apparatus. a request receiving unit configured to receive the request packet transmitted by the request transmitting unit, and a response transmitting unit configured to: generate the response packet in response to the request packet received by the request receiving unit; when the request packet received by the request receiving unit includes a data write request or an interrupt request with respect to a process of the second apparatus, requiring a sequential order of request processing, generate the redundant response packet; and transmit the generated response packet and, when the redundant response packet has been generated, transmit the generated redundant response packet, to the first apparatus. generate the response packet in response to the request packet received by the request receiving unit; when the request packet received by the request receiving unit includes a data write request or an interrupt request with respect to a process of the second apparatus, requiring a sequential order of request processing, generate the redundant response packet; and transmit the generated response packet and, when the redundant response packet has been generated, transmit the generated redundant response packet, to the first apparatus.