PURPOSE: To enable the conservation of labor required for the monitoring and cleaning of coagulated ash adhering to a horn tip part by adding a delay circuit and a voltmeter for generating a signal delayed by propagation time of sound wave reciprocating to a horn to the device for measuring temperature in a fire furnace utilizing sound velocity.

CONSTITUTION: This apparatus is provided with a delay circuit 11 to input an output of an amplifier 7 and an output of a preamplifier 8, a voltmeter 12 and an alarm generator 13. When coagulated ash 10 adheres to the tip of a transmission horn 4, a part of a sound wave emitted from a transmitter 2 is reflected to be returned to the transmitter 2 and a sound pressure thereof is converted to a voltage to generate an echo signal. In addition, on the side of a converter 3, a sound wave reached is reflected with a receiver 3, further on the coagulated ash 10 at the tip of a horn 5 and received 3 again to emit an echo signal. Time τ1 and τ2 of transmitting and receiving waves in the reciprocation between horns 4 and 5 is almost equal. This enables the monitoring of the size of the echo signal. namely, the degree of the adhesion of the coagulated ash 10 by operating the voltmeter 12 by an output of the delay circuit 11 adapted to generate a delay time by the τ1.