To provide an ultrasonic oscillator suitable for a flow measuring instrument using highly precise sing-around measurement.

This oscillator is constituted of a metal cap 2 of a topped cylindrical shape having a top part and a sidewall part, a sound-matching layer 3 provided in an outer face of the top part, and a piezoelectric member 5 fixed adhesively to an inner face of the top part, an elastic body 13 is embedded between a sidewall of the piezoelectric member 5 and the cap 2, and a leading characteristic or a tailing characteristic of the ultrasonic oscillator thus gets sufficient even when used for an ultrasonic flowmeter using a sing-around method.