The present invention relates to a non-invasive early diagnostic method and device that allows an early diagnosis of Marssonina blotch disease, which infects apple tree leaves, before the occurrence of lesions to enable early prevention of apple Marssonina blotch disease. The method and device of the present invention comprise the following steps: irradiating a broadband light source on an apple tree leaf undergoing diagnosis by means of optical coherence tomography; detecting interference signals from light reflected from the apple tree leaf undergoing diagnosis and light reflected from a reference arm; signal processing the interference signals, which have been detected through optical coherence tomography by means of a signal processing unit; obtaining digital tomographic image data on the apple tree leaf undergoing diagnosis; analyzing the digital tomographic image by means of a diagnostic unit; and observing the boundary of at least two tissues, wherein the apple tree leaf undergoing diagnosis is considered to be healthy and uninfected with Marssonina blotch disease if the boundary between at least two tissues is clearly defined, and wherein the apple tree leaf undergoing diagnosis is considered to be infected with Marssonina blotch disease if the boundary between at least two tissues is not clearly defined.