This invention offers a signal-off detection circuit allowing arbitrary setting of an issuing time (response time) of a signal disconnection alarm without being affected by a time constant of a direct current feedback circuit giving an offset voltage to an amplifier for amplifying a data signal. Input data signals per a fixed time determined by a timer is counted by a counter, and a count value is compared with a predetermined set value in a comparator. A configuration is made such that a signal disconnection alarm may be issued by detecting a disconnection state of the data signal according to a comparison result. Thereby, an issuing time of a signal disconnection alarm can be set without being affected by a time constant of a direct current feedback circuit giving an offset voltage to an amplifier for amplifying a data signal of a preceding stage.