1. A medical imaging apparatus comprising: a positioning image acquisition unit which acquires a positioning image concerning a subject; a unit which sets an region of interest in the positioning image in accordance with specification performed by an operator; a monitoring image acquisition unit which repeatedly acquires a monitoring image concerning the subject at predetermined time intervals; and a generation unit which combines an region-of-interest image representing the region of interest with the monitoring image to generate a display image every time the monitoring image is acquired.