A delay measuring device of a communication system that includes the delay measuring device sequentially transmitting and receiving frames having a known frame length, and the communication system further includes a measurement object device serving as an object for measuring a round trip time by the delay measuring device, the delay measuring device comprising: a round trip time measuring unit to measure a round trip time with respect to the measurement object device a number of times using information for delay measurement in the frame; a round trip time change detection unit to detect a change of a value of the round trip time, based on a plurality of values of the round trip time thus measured; a round trip time determination unit to determine a value of the round trip time to be adopted, from among the values of the round trip time, based on the change of a value of the round trip time; characterized by the round trip time change detection unit being configured to calculate a frequency deviation of frame transmission frequencies between the delay measuring device and the measurement object device from the change of a value of the round trip time based on two values of the round trip time having a state where values of the round trip time continuously decrease or increase; and a measurement resolution determination unit to determine a round trip time measurement resolution based on the frequency deviation calculated from the change of a value of the round trip time; and the delay measuring device having a function of changing a transmission frequency or phase of the frames or instructing the measurement object device to change a transmission frequency or phase of the frames by inserting an empty bit or empty bits between frames a number of times while changing the empty bit size, based on the frequency deviation and the round trip measurement resolution. a round trip time measuring unit to measure a round trip time with respect to the measurement object device a number of times using information for delay measurement in the frame; a round trip time change detection unit to detect a change of a value of the round trip time, based on a plurality of values of the round trip time thus measured; a round trip time determination unit to determine a value of the round trip time to be adopted, from among the values of the round trip time, based on the change of a value of the round trip time; characterized by the round trip time change detection unit being configured to calculate a frequency deviation of frame transmission frequencies between the delay measuring device and the measurement object device from the change of a value of the round trip time based on two values of the round trip time having a state where values of the round trip time continuously decrease or increase; and a measurement resolution determination unit to determine a round trip time measurement resolution based on the frequency deviation calculated from the change of a value of the round trip time; and the delay measuring device having a function of changing a transmission frequency or phase of the frames or instructing the measurement object device to change a transmission frequency or phase of the frames by inserting an empty bit or empty bits between frames a number of times while changing the empty bit size, based on the frequency deviation and the round trip measurement resolution.