A flow measuring instrument of fluid, the instrument comprising a first oscillator disposed at a fluid duct for transmitting an ultrasonic signal; a second oscillator for receiving the ultrasonic signal transmitted from the first oscillator; a time measuring section for finding a travel time of an ultrasonic wave by carrying out one travel or a plurality of travels in series of the ultrasonic wave between the first and the second oscillators (2, 3); a calculation section for finding a flow velocity or a flow volume based on the travel time measured by the time measuring section; a high rate clock generator working at a high frequency; a low rate clock generator working at a low frequency; a clock control section for controlling an operation of a high rate clock; a measurement control section, working based on a high rate clock supplied from the high rate clock generator and a low rate clock supplied from the low rate clock generator, for controlling the first and the second oscillators and a time measuring operation done by the time measuring section; the instrument being characterized in that it further comprises an auxiliary clock generator whose clock works at a higher rate than the high rate clock, wherein the auxiliary clock generator supplies an auxiliary clock only at a part of the time measuring operation done by the time measuring section, the travel time (Tx) is obtained by using both a first travel time (Ty) measured using the high rate clock and a second travel time (Tz) measured using the auxiliary clock, the auxiliary clock generator supplies the auxiliary clock when the second oscillator receives the ultrasonic signal transmitted from the first oscillator and stops the auxiliary clock when the first time (Ty) is measured, and when the time measuring section finishes the time measuring operation, the clock control section stops a high rate clock supplied from the high rate clock generator. a first oscillator disposed at a fluid duct for transmitting an ultrasonic signal; a second oscillator for receiving the ultrasonic signal transmitted from the first oscillator; a time measuring section for finding a travel time of an ultrasonic wave by carrying out one travel or a plurality of travels in series of the ultrasonic wave between the first and the second oscillators (2, 3); a calculation section for finding a flow velocity or a flow volume based on the travel time measured by the time measuring section; a high rate clock generator working at a high frequency; a low rate clock generator working at a low frequency; a clock control section for controlling an operation of a high rate clock; a measurement control section, working based on a high rate clock supplied from the high rate clock generator and a low rate clock supplied from the low rate clock generator, for controlling the first and the second oscillators and a time measuring operation done by the time measuring section; the instrument being characterized in that it further comprises an auxiliary clock generator whose clock works at a higher rate than the high rate clock, the auxiliary clock generator supplies an auxiliary clock only at a part of the time measuring operation done by the time measuring section, the travel time (Tx) is obtained by using both a first travel time (Ty) measured using the high rate clock and a second travel time (Tz) measured using the auxiliary clock, the auxiliary clock generator supplies the auxiliary clock when the second oscillator receives the ultrasonic signal transmitted from the first oscillator and stops the auxiliary clock when the first time (Ty) is measured, and when the time measuring section finishes the time measuring operation, the clock control section stops a high rate clock supplied from the high rate clock generator.