An ultrasonic transducer element chip (17, 17a, 17b) comprising: a substrate including openings arranged in an array pattern; ultrasonic transducer elements respectively disposed at the openings on a first surface of the substrate; a reinforcing member fixed on a second surface of the substrate opposite to the first surface of the substrate to reinforce the substrate, the reinforcing member including linear grooves (53, 84, 85, 86) formed on a surface of the reinforcing member fixed on the second surface of the substrate so that internal spaces of the openings and an external space of the substrate are in communication with each other via the linear grooves (53, 84, 85, 86) to ensure ventilation, the linear grooves (53, 84, 85, 86) extending along a plane of the surface of the reinforcing member, and the linear grooves (53, 84, 85, 86) being arranged at an interval in a first direction smaller than a width of each of the openings on the second surface of the substrate in the first direction, characterized in that in a plan view seen along a substrate thickness direction, either each of the linear grooves (53, 84, 85, 86) continuously extends, or a combination of a plurality of the linear grooves (53, 84, 85, 86) extends, across each of the openings in a corresponding one of the columns of the array pattern so that the internal spaces of adjacent ones of the openings in the corresponding one of the columns of the array pattern are in communication with each other and the internal space of the one of the openings disposed at an end of the corresponding one of the columns is in communication with the external space disposed outside of an outline of the substrate. a substrate including openings arranged in an array pattern; ultrasonic transducer elements respectively disposed at the openings on a first surface of the substrate; a reinforcing member fixed on a second surface of the substrate opposite to the first surface of the substrate to reinforce the substrate, the reinforcing member including linear grooves (53, 84, 85, 86) formed on a surface of the reinforcing member fixed on the second surface of the substrate so that internal spaces of the openings and an external space of the substrate are in communication with each other via the linear grooves (53, 84, 85, 86) to ensure ventilation, the linear grooves (53, 84, 85, 86) extending along a plane of the surface of the reinforcing member, and the linear grooves (53, 84, 85, 86) being arranged at an interval in a first direction smaller than a width of each of the openings on the second surface of the substrate in the first direction, characterized in that in a plan view seen along a substrate thickness direction, either each of the linear grooves (53, 84, 85, 86) continuously extends, or a combination of a plurality of the linear grooves (53, 84, 85, 86) extends, across each of the openings in a corresponding one of the columns of the array pattern so that the internal spaces of adjacent ones of the openings in the corresponding one of the columns of the array pattern are in communication with each other and the internal space of the one of the openings disposed at an end of the corresponding one of the columns is in communication with the external space disposed outside of an outline of the substrate.