A rotary damper 1 includes a synthetic resin-made accommodating case 4 for accommodating in its interior 2 a viscous fluid 3 which is constituted by silicone oil or the like and whose viscosity declines with a temperature rise, and a synthetic resin-made rotor 9 which is disposed in the interior 2 of the accommodating case 4 rotatably, i. The invention estimates the modulated data actually transmitted by the network termination modems from the frequency components of the discrete multitone symbols received by the line termination modems and to evaluate the far-end crosstalk as a linear combination of these estimates. , is in an ON condition, by outputting the alarm time itself.