An ultrasonic apparatus for measuring the concentration and flow rate of a sample gas, comprising: a conduit for flowing the sample gas; a first ultrasonic transmission-reception device mounted to the inside of the conduit; a second ultrasonic transmission-reception device mounted to the inside of the conduit to face the first ultrasonic transmission-reception device; a transmission-reception switch for switching the operation mode of the first and second ultrasonic transmission-reception devices between a transmission mode for transmitting ultrasonic waves and a reception mode for receiving ultrasonic waves; a temperature sensor, disposed in the conduit, for measuring the temperature of the sample gas flowing through the conduit; the first ultrasonic transmission- reception device generating forward ultrasonic waves relative to the flow direction of the sample gas when the device is in the transmission mode and generating backward waveform when the device is in the reception mode on the basis of the received ultrasonic waves generated by the second ultrasonic transmission-reception device; the second ultrasonic transmission-reception device generating backward ultrasonic waves relative to the flow direction of the sample gas when the device is in the transmission mode and generating forward waveform when the device is in the reception mode on the basis of the received ultrasonic waves generated by the first ultrasonic transmission-reception device; means for generating trigger signals when the forward and backward waveforms pass over a predetermined level; means for generating forward and backward zero-cross signals when the forward and backward waveforms pass over a zero level; propagation time calculation means, coupled to the temperature sensor, the trigger signal generating means and the zero-cross signal generating means, for calculating a possible propagation time range on the basis of the gas temperature detected by the temperature sensor, determining whether or not the phases at which two first trigger signals, respectively generated on the basis of the forward and backward waveforms, coincide with each other, processing the zero-cross signals so that the phases coincide with each other if they do not coincide with each other, obtaining reference zero-cross time instant by calculating mean value of the forward and backward zero-cross time instants, obtaining an ultrasonic reception point by subtracting an integral multiple of the cycle of the ultrasonic waves so that the results of the subtraction falls into the possible propagation time range and estimating the ultrasonic propagation time on the basis of the ultrasonic reception point. a conduit for flowing the sample gas; a first ultrasonic transmission-reception device mounted to the inside of the conduit; a second ultrasonic transmission-reception device mounted to the inside of the conduit to face the first ultrasonic transmission-reception device; a transmission-reception switch for switching the operation mode of the first and second ultrasonic transmission-reception devices between a transmission mode for transmitting ultrasonic waves and a reception mode for receiving ultrasonic waves; a temperature sensor, disposed in the conduit, for measuring the temperature of the sample gas flowing through the conduit; the first ultrasonic transmission- reception device generating forward ultrasonic waves relative to the flow direction of the sample gas when the device is in the transmission mode and generating backward waveform when the device is in the reception mode on the basis of the received ultrasonic waves generated by the second ultrasonic transmission-reception device; the second ultrasonic transmission-reception device generating backward ultrasonic waves relative to the flow direction of the sample gas when the device is in the transmission mode and generating forward waveform when the device is in the reception mode on the basis of the received ultrasonic waves generated by the first ultrasonic transmission-reception device; means for generating trigger signals when the forward and backward waveforms pass over a predetermined level; means for generating forward and backward zero-cross signals when the forward and backward waveforms pass over a zero level; propagation time calculation means, coupled to the temperature sensor, the trigger signal generating means and the zero-cross signal generating means, for calculating a possible propagation time range on the basis of the gas temperature detected by the temperature sensor, determining whether or not the phases at which two first trigger signals, respectively generated on the basis of the forward and backward waveforms, coincide with each other, processing the zero-cross signals so that the phases coincide with each other if they do not coincide with each other, obtaining reference zero-cross time instant by calculating mean value of the forward and backward zero-cross time instants, obtaining an ultrasonic reception point by subtracting an integral multiple of the cycle of the ultrasonic waves so that the results of the subtraction falls into the possible propagation time range and estimating the ultrasonic propagation time on the basis of the ultrasonic reception point.