Both a first profile that represents relationships between delay times premeasured over transmission paths and occurrence frequencies of the delay times and measurement periods of time for which the delay times over the transmission paths are measured are stored such that the first profile and the measurement period of time are correlated; a measurement period of time correlated with the first profile is obtained from the storage section if a second profile that represents relationships between delay times measured to obtain the measurement period of time and occurrence frequencies of the delay times is the same profile as the first profile; the delay times over the transmission paths are measured; and a mean value of the delay times measured for the measurement period of time is computed.