An optical transmission system that performs optical transmission over an optical switching network, comprising: a packet generation unit adapted to store data received from a data transmission source on a memory and generate a plurality of packets based on the data; a transmitter unit(111,112,113,114) adapted to change the plurality of packets into a plurality of optical packets; an optical wavelength-multiplexing unit adapted to generate a multiplexed optical-packet signal by performing optical wavelength multiplexing on a the plurality of optical packets, and transmit the multiplexed optical-packet signal to a transmission destination; a retransmitting unit (211,212,213,214,311,312,313,314) adapted to retransmit the multiplexed optical-packet signal transmitted from the optical wavelength-multiplexing unit back to a transmission source of the transmitted multiplexed optical-packet signal, after the multiplexed optical-packet signal has passed the optical switching network; a skew-amount detecting unit adapted to detect a skew amount of the optical packet allocated to each wavelength band of the multiplexed optical-packet signal retransmitted from the retransmitting unit; characterised in that the packet generation unit is adapted to adjust, based on the skew amount detected by the skew-amount detecting unit, a delay amount of an optical packet allocated to each wavelength band of a multiplexed optical-packet signal transmitted to a transmission destination by the optical wavelength-multiplexing unit, wherein the packet generation unit is adapted to request to the data transmission source for discontinuing data output to the packet generation unit when an amount of data stored in the memory exceeds a reference value, and the packet generation unit is adapted to request to the data transmission source for resuming data output to the packet generation unit when the amount of data stored in the memory is less than the reference value. a packet generation unit adapted to store data received from a data transmission source on a memory and generate a plurality of packets based on the data; a transmitter unit(111,112,113,114) adapted to change the plurality of packets into a plurality of optical packets; an optical wavelength-multiplexing unit adapted to generate a multiplexed optical-packet signal by performing optical wavelength multiplexing on a the plurality of optical packets, and transmit the multiplexed optical-packet signal to a transmission destination; a retransmitting unit (211,212,213,214,311,312,313,314) adapted to retransmit the multiplexed optical-packet signal transmitted from the optical wavelength-multiplexing unit back to a transmission source of the transmitted multiplexed optical-packet signal, after the multiplexed optical-packet signal has passed the optical switching network; a skew-amount detecting unit adapted to detect a skew amount of the optical packet allocated to each wavelength band of the multiplexed optical-packet signal retransmitted from the retransmitting unit; characterised in that the packet generation unit is adapted to adjust, based on the skew amount detected by the skew-amount detecting unit, a delay amount of an optical packet allocated to each wavelength band of a multiplexed optical-packet signal transmitted to a transmission destination by the optical wavelength-multiplexing unit, wherein the packet generation unit is adapted to request to the data transmission source for discontinuing data output to the packet generation unit when an amount of data stored in the memory exceeds a reference value, and the packet generation unit is adapted to request to the data transmission source for resuming data output to the packet generation unit when the amount of data stored in the memory is less than the reference value.