PURPOSE: To obtain an ultrasonic sensor in which the detection area can be made thin easily by fixing an independent horn detachably to the front of a hollow spherically molded item having a horn part for guiding an ultrasonic wave transmitted from an ultrasonic oscillator or received thereby.

CONSTITUTION: A tubular part 32 is provided at the forward end part of a hollow spherical molded item 30 for housing an ultrasonic oscillator 10 and provided with a horn part 31 for guiding an ultrasonic wave transmitted therefrom or received thereby. Recesses 33, 33 are made oppositely in the tubular part 32 and an annular recess 34 is made on the base side thereof. An independent horn 90 being fixed detachably to the front end part of the molded item 30 is provided oppositely with two projecting leaf spring parts 92. Hemispheric protrusions 93, 93 being fitted in the recesses 33, 33 in the tubular part 32 are provided on the inner side face at the leaf spring part 92.