A system for mutually synchronizing a first clock signal placed in a first network element and a second clock signal placed in a second network element, said first network element being arranged to transmit synchronizing messages to said second network element, and said second network element being arranged to receive said synchronizing messages, said system comprising: - calculation means that is arranged to form a difference value of a synchronizing message, said difference value being essentially a difference of a reception time value and a transmission value of the synchronizing message, said reception time value depending on a cumulated number of periods of the second clock signal at the moment of arrival of the synchronizing message at a predetermined synchronizing point in the second network element, and said transmission value depending on a position of said synchronizing message in a chronological transmission order of the synchronizing messages, and - adjusting means that is arranged to adjust the second clock signal on the basis of information contained by difference values corresponding to chronologically successive synchronizing messages, characterized in that said adjusting means is arranged to: - determine a first area of a margin of fluctuation of the difference values and a second area of the margin of fluctuation of the difference values so that the first area represents smaller difference values than the second area, the first area represents the lowest part of the margin of fluctuation of the difference values, and a predetermined share of the difference values belong to the first area, and - weight, when adjusting the second clock signal on the basis of the information contained by the difference values, the information contained by the difference values belonging to the first area of the margin of fluctuation of the difference values more heavily than the information contained by the difference values belonging to the second area of the margin of fluctuation of the difference values. - calculation means that is arranged to form a difference value of a synchronizing message, said difference value being essentially a difference of a reception time value and a transmission value of the synchronizing message, said reception time value depending on a cumulated number of periods of the second clock signal at the moment of arrival of the synchronizing message at a predetermined synchronizing point in the second network element, and said transmission value depending on a position of said synchronizing message in a chronological transmission order of the synchronizing messages, and - adjusting means that is arranged to adjust the second clock signal on the basis of information contained by difference values corresponding to chronologically successive synchronizing messages, - determine a first area of a margin of fluctuation of the difference values and a second area of the margin of fluctuation of the difference values so that the first area represents smaller difference values than the second area, the first area represents the lowest part of the margin of fluctuation of the difference values, and a predetermined share of the difference values belong to the first area, and - weight, when adjusting the second clock signal on the basis of the information contained by the difference values, the information contained by the difference values belonging to the first area of the margin of fluctuation of the difference values more heavily than the information contained by the difference values belonging to the second area of the margin of fluctuation of the difference values.