A storage unit (106) stores pieces of information regarding intensities of ultrasound waves detected by multiple oscillators (101) in time series. The multiple oscillators (101) receives an ultrasound wave reflected on an object (107), to which a second ultrasound wave (108) generated by an ultrasound probe (102) is focused, in order from an oscillator closest to the object (107). The second ultrasound wave (108) is transmitted in a direction different from that of a first ultrasound wave (103). Information regarding an intensity of the ultrasound wave reflected on the object (107), which is obtained by storing in the storage unit (106) in order from the oscillator closest to the object (107), is obtained. This information is subtracted from the pieces of information stored in the storage unit (106). Thus, a received signal from a side lobe is extracted to be subtracted from a whole received signal.