The present invention generally relates to a medical apparatus. More particularly, the present invention relates to an apparatus for delivering a medium to body cavity of mammals including a human, especially to an apparatus for expanding rectum. Specifically, the present invention relates to the field of focused ultrasound treatment with a main purpose to provide an auxiliary device for ultrasound therapy applied to uterine myoma so as to avoid damage by ultrasound after focused within body. Therefore, the present invention provides an apparatus for blocking ultrasound, comprising (1) lead-in means for entering into said body cavity; and (2) accommodation means for containing ultrasound blocking medium, which enwraps the lead-in means outside.