To provide a differential interference microscope most suited to application of patch clamp method by arranging either an objective lens for illumination or an objective lens for observation above a specimen and constituting the objective lens for illumination or the objective lens for observation to be arranged above the specimen with an objective lens for immersion.

A cultured cell 8 is immersed in a culture fluid 10 filled in a laboratory dish 9 and moreover, an objective lens for immersion 6 is arranged so that the tip part of it is immersed in the culture fluid 10. Moreover, the luminous flux transmitting the cultured cell 8 to be illuminated by the objective lens for immersion 6 is condensed by an objective lens for observation 11 having the same focal length as that of the objective lens for immersion 6 to be projected on a primary condensing position 14 with a Normarski prism 12 and an analyzer 13 and a magnified image at here is condensed on a secondary condensing position 16 by being relaid by relay lenses 15. Then, the magnified image at the secondary condensing position 16 is made observable with the naked eye 18 through an eyepiece 17.