Concurrent injury of medial malleolus and deltoid ligament is difficult to occur considering the injury mechanism. When the concurrent injury comes about, the deltoid ligament injury could be missed and it may lead to medial ankle instability. There are few reported cases of the concurrent injury and domestic case of concurrent failure of both structures over the medial side has been reported just once; however, the injury mechanism is different from this case. The authors report a case of medial malleolus fracture with deltoid ligament rupture following pronation injury with a review of necessity of repairing deltoid ligament for ankle stability.