Transmission system with congestion state-based flow control

A transmission system can perform flow control even if a flow control instruction is invalidated, and even when the transmission system malfunctions, it can autonomously recover to normal operation for communications. A flow control instruction sender has at least one of two functions including a normal transmission mode to send a flow control instruction for flow control over a congestion state if an event, such as the occurrence of a congestion or the removal of a congestion, occurs when a data receiver receives data, and an autonomous transmission mode to select one of a plurality of congestion states at present in each period and autonomously send a flow control instruction corresponding to the selected congestion state. The flow control instruction sender preferentially sends a flow control instruction in the normal transmission mode in which a congestion actually occurs or is actually removed, when timings of the normal transmission mode and the autonomous transmission mode coincide with each other. A flow control instruction receiver receives a flow control instruction to perform flow control by stopping sending data and sending data.