An imaging apparatus is disclosed for diagnosis including a plurality of transmitting and receiving units, an error of a scale of a tomographic image to be generated is reduced. The imaging apparatus includes a transmitting and receiving unit comprising a first transmitting and receiving unit configured to perform transmission and reception of an ultrasound signal and a second transmitting and receiving unit configured to perform transmission and reception of an optical signal when rotating in the lumen of the measurement subject body; and a processor configured to: convert positional information of each position outside the range in which the flushing liquid flows regarding the ultrasound line data generated by the processor based on a ratio between the ultrasound propagation velocity in the flushing liquid and the ultrasound propagation velocity in the blood vessel; and construct the first tomographic image using the ultrasound line data which is converted by the processor.