What is the cause of frequent urination. blood sugar is normal & no evident of any kind of infection?

Head injury?: Hi. You've ruled out the two most common causes, diabetes and UTI. Have you had a head injury. Rare as it is, your doc must at least consider diabetes insipidus (DI, an acquired disease of lack of antidiuretic hormone). If your doc suspects that, it's straightforward to test for with a water deprivation test, but the test needs to be closely monitored. DI can follow head injury or be idiopathic.