We measured virus retention by many types of membranes including microfiltration membranes, ultrafiltration membranes and nanofiltration membranes. We succeeded in evaluating quantitatively virus retention in a very high retention range by employing coliphage Q beta and T4 as model viruses. Q beta, which is the smaller virus in this study, penetrated all tested pieces of ultrafiltration membranes and nanofiltration membranes, though the retention was very high such as in the range of 99-99.9999%. The analysis of the polyethylene glycol retention data has shown that the leakage of viruses is caused by abnormally larger pores which are not included in the main pore size distribution. The diameter of the abnormal pores was estimated from the results of retention of different coliphages. The leakage of Q beta was also observed in the case of inorganic ceramic ultrafiltration membranes, but T4, which is larger than Q beta, cannot penetrate them. Some types of microfiltration membrane have shown higher retention than ultrafiltration membranes and nanofiltration membranes. This suggests the possibility that we can develop high virus retention membranes with low filtration resistance.