  The effect of extremely low frequency primordial gravitational wave with
arbitrary direction of propagation on a non-aligned gravitational lens system
is investigated. From the point of view of real astrophysical lens model,
singular isothermal sphere lens model is adopted in the gravitational lens
system. The results show that, under the perturbation from extremely low
frequency primordial gravitational wave, time delay in the gravitational lens
system could strongly deviate from that deduced from theoretical model. This
means that the strongly deviate time delay could be the imprint of extremely
low frequency primordial gravitational wave on gravitational lens system,
indicating that gravitational lens system could be used as a long baseline
detector to detect extremely low frequency primordial gravitational wave.
