Sacroiliac joint (SIJ) can cause low back pain when its joint capsule and ligamentous tissue are damaged. We report our experience in treating three SIJ dysfunction patients presenting with acute low back pain (a 38 year-old male, a 24 year-old male, and a 32 year-old female). SIJ dysfunction was diagnosed using the one-finger test, the modified Newton test, and SIJ injection. In all three patients, lumbar MRI demonstrated slightly degenerated lumbar lesions (lumbar canal stenosis, lumbar disc hernia). Two patients had paresthesia or pain in the leg and all three patients showed iliac muscle tenderness in the groin, which was thought to be a referred symptom because of improvement after SIJ injection. The two male patients returned to work and the problems have not recurred. Although our female patient resumed daily life as a housewife, her condition recurred at intervals of 2-3 months and she required regular SIJ injections. The prevalence of SIJ dysfunction of low back pain is about 10%, so it should be considered as a differential diagnosis when treating low back pain and designing treatment for lumbar spinal disorders.