Oliguria is mainly defined as less than 400 ml of urine in 24 hours or less than 17 ml of urine per hour for statistical purposes. There are many reasons for oliguria, and it is often necessary to determine whether there is shock, hemorrhage, severe dehydration, cardiac insufficiency, renal artery embolism, nephrotic syndrome, burns, or a history of severe renal disease as the primary cause. In addition, there is no urinary tract obstruction, prostate enlargement, stones, pelvic tumor or urinary tract injury, neurogenic bladder, etc. If oliguria is accompanied by bleeding, it is usually seen in blood loss or blood loss caused by trauma; oliguria with large amount of proteinuria is usually seen in nephrotic syndrome; oliguria with severe underlying liver disease is seen in hepatorenal syndrome; oliguria with hematuria, hypertension, swelling, etc. is easily seen in various acute nephritis and acute progressive nephritis; oliguria with back pain and urinary pain is seen in urinary stones. The reason for this is that it is a very good idea to have a good understanding of the situation. The causes of oliguria can be summarized as reduced renal blood flow, reduced glomerular filtration rate, damage to glomerular and tubular function due to renal parenchymal lesions, and some urinary tract obstruction caused by various reasons.