[Purpose] This study aimed to quantitatively analyze characteristics of and changes in internal muscle structure according to the time of delayed onset muscle soreness (DOMS) using ultrasound imaging, thereby presenting clinical evidential data for evaluation of muscle damage. [Subjects] We recruited 38 male subjects. [Methods] Ultrasound images of the medial gastrocnemius muscle prior to induction of DOMS and immediately after, 24 hours after, 48 hours after, and 72 hours after induction of DOMS were obtained, and the thickness and pennation angle of the muscle were measured. [Results] The muscle thickness gradually increased until 48 hours after induction of DOMS and decreased after 72 hours. The pennation angle also gradually increased until 48 hours after induction of DOMS and decreased after 72 hours. [Conclusion] Ultrasound imaging is considered useful for assessment of structural characteristics of muscles when muscle damage like DOMS takes place.