[Claim 1] A communication system comprising:
a master device and a slave device connected via a relay device whose relay delay is not constant,
wherein the master device includes
a master counter which is a free-run counter; and
a transmission unit, at a timing for transmitting a time synchronization frame for time synchronization, to acquire current time as transmission time of the time synchronization frame, to acquire a count value of the master counter as a transmission count value, to store the transmission time and the transmission count value in the time synchronization frame, and to transmit the time synchronization frame to the slave device, and
wherein the slave device includes
a slave counter which is a free-run counter;
a reception unit to receive the time synchronization frame;
an acquisition unit to acquire reception time of the time synchronization frame and a reception count value which is a count value of the slave counter at the reception time; and
a time correction unit to correct time of the slave device, using the transmission time, the transmission count value, the reception time, the reception count value, a measurement propagation delay value and a delay count difference, the measurement propagation delay value being a value of propagation delay acquired from a past measurement, the delay count difference being obtained from a count value of the master counter and a count value of the slave counter which correspond to the

measurement propagation delay value.
[Claim 2] The communication system according to claim 1,
wherein the time correction unit calculates, as a relay delay difference, a difference between a value of relay delay by the relay device when relaying the time synchronization frame and a value of relay delay included in the measurement propagation delay value, based on the transmission count value, the reception count value and the delay count difference, and
corrects the time of the slave device, using the calculated relay delay difference. I [Claim 3] The communication system according to claim 2,
wherein the time correction unit calculates a value of propagation delay at transmission of the time synchronization frame, using the relay delay difference and the measurement propagation delay value, and
corrects the time of the slave device, using the calculated value of propagation » delay at the transmission of the time synchronization frame. [Claim 4] The communication system according to claim 1,
wherein the time correction unit calculates, as an offset value, a difference between the time of the master device and the time of the slave device, using a calculated value of propagation delay at the transmission of the time synchronization ) frame, the transmission time and the reception time, and
corrects the time of the slave device, using the calculated offset value. [Claim 5] The communication system according to claim 2,
wherein the time correction unit calculates the relay delay difference based on the transmission count value, the reception count value, the delay count difference, a > clock frequency of the master device and a clock frequency of the slave device.

[Claim 6] The communication system according to claim 1,
wherein the time correction unit corrects the time of the slave device, using an average value of a plurality of values of relay delay acquired in a plurality of past measurements as the measurement propagation delay value. [Claim 7] The communication system according to claim 6,
wherein the time correction unit uses, as the measurement propagation delay value, the average value of the plurality of values of propagation relay measured by transmitting and receiving the frames to and from the master device. [Claim 8] A master device connected to a slave device via a relay device whose relay delay is not constant, the master device comprising:
a master counter which is a free-run counter; and a transmission unit, at a timing for transmitting a time synchronization frame for time synchronization, to acquire current time as transmission time of the time synchronization frame, to acquire a count value of the master counter as a transmission count value, to store the transmission time and the transmission count value in the time synchronization frame, and to transmit to the slave device, the time synchronization frame in which the transmission time and the transmission count value are stored. [Claim 9] A slave device connected to a master device via a relay device whose relay delay is not constant, the master device including a master counter which is a free-run counter, and at a timing for transmitting a time synchronization frame for time synchronization, acquiring current time as transmission time of the time synchronization frame, acquiring a count value of the master counter as a transmission count value, storing the transmission time and the transmission count value in the time synchronization frame, and transmitting the time synchronization frame in which the transmission time and the transmission count value are stored, the slave device

comprising:
a slave counter which is a free-run counter;
a reception unit to receive the time synchronization frame transmitted from the master device;
an acquisition unit to acquire reception time of the time synchronization frame and a reception count value which is a count value of the slave counter at the reception time; and
a time correction unit to correct time of the slave device, using the transmission time, the transmission count value, the reception time, the reception count value, a measurement propagation delay value and a delay count difference, the measurement propagation delay value being a value of propagation delay acquired from a past measurement, the delay count difference being obtained from a count value of the master counter and a count value of the slave counter which correspond to the measurement propagation delay value. [Claim 10] The slave device according to claim 9,
wherein the time correction unit calculates, as a relay delay difference, a difference between a value of relay delay by the relay device when relaying the time synchronization frame and a value of relay delay included in the measurement propagation delay value, based on the transmission count value, the reception count value and the delay count difference, and
corrects the time of the slave device, using the calculated relay delay difference. [Claimll] The slave device according to claim 10,
wherein the time correction unit calculates a value of propagation delay at transmission of the time synchronization frame, using the relay delay difference and the

measurement propagation delay value, and
corrects the time of the slave device, using the calculated value of propagation delay at transmission of the time synchronization frame. [Claim 12] The slave device according to claim 9,
wherein the time correction unit calculates, as an offset value, a difference between the time of the master device and the time of the slave device, using a calculated value of propagation delay at transmission of the time synchronization frame, the transmission time and the reception time, and
corrects the time of the slave device, using the calculated offset value. [Claim 13] The slave device according to claim 10,
wherein the time correction unit calculates the relay delay difference based on the transmission count value, the reception count value, the delay count difference, a clock frequency of the master device and a clock frequency of the slave device. [Claim 14] The slave device according to claim 9,
wherein the time correction unit corrects the time of the slave device, using an average value of a plurality of values of relay delay acquired in a plurality of past measurements as the measurement propagation delay value. [Claim 15] The slave device according to claim 14,
wherein the time correction unit uses, as the measurement propagation delay value, the average value of the plurality of values of propagation relay measured by transmitting and receiving the frames to and from the master device.