PURPOSE: To detect a period where the motion is smallest among periodic respiration motions and to obtain bright images which are decreased in the artifact of the bodily movements by determining the correction time obtd. after the respiration period is divided by an integer and adding the correction time to the level signals of the inhalation state and exhalation state obtd. from respiration waveforms.

CONSTITUTION: The output G of an MM 21 has the pulse width corresponding to the period from the inhalation start of the respiration waveform signals to the next inhalation start. A counter 26 counts the clocks K past an AND circuit 24. This count value N further passes a division circuit 27 which is previously set with a divisor with a divisor setting switch circuit 28. The output of the divider circuit 27 is latched and is counted down. The output M thereof is the signal detecting the period of the smallest motion among the period motions of the respiration. The output M is the signal detecting only the second half of the inhalation state if the set number of the divisor setting switch 28 of the divider circuit 27 is set at, for example, 4. A computer executes the constitution of the images for very echo signal in accordance with the pulse sequence based on the signal from this counter circuit 16.