[Purpose] Determining the thickness of the intercostal muscle with ultrasound imaging would be a useful parameter in evaluating respiratory muscle activity in patients with tetraplegia and neuromuscular weakness. However, it has not been clarified whether ultrasound imaging can measure changes in intercostal muscle thickness during breathing. This study aimed to measure contractions of the human intercostal muscle in the anterior, lateral, and posterior parts with ultrasound imaging during maximal breathing. [Participants and Methods] The participants were 12 healthy males. Intercostal muscle thickness was measured using ultrasound at rest and at maximal breathing. The measurement sites were the anterior, lateral, and posterior portions of the right intercostal spaces. Statistical analysis was performed using a paired t-test comparing intercostal muscle thickness at rest and maximal breathing. [Results] The thickness of the intercostal muscle showed significant increases in the first, second, third, fourth, and sixth intercostal spaces of the anterior portions. There were no significant differences in the lateral or posterior portions between rest and maximal breathing. [Conclusion] Human intercostal muscle thickness can be measured with ultrasound and increases only in the anterior portions during maximal breathing.