Collimator is popular used in optical testing in laboratory to simulate target in a distance. Glass plate with pattern is fundamental constitute of collimator focal plane, which should be placed precisely with respect to its optics. For collimator in used or to be built, it is necessary to check position of its focal plane. We develop a fast method to check focal plane position with interferometer. Results derived from this method are compared with that derived from angle deviation measurement method. Meanwhile, focal plane adjustment of imaging telescope can be performed with same way.