The present disclosure relates to a use of JULGI in diagnosing and treating guanine quadruplex-related disease and, more particularly, to a guanine-quadruplex (G-quadruplex) detecting composition comprising JULGI protein or a protein having a homology of 80% or higher to the JULGI protein, a diagnostic composition comprising the detecting composition, a diagnostic kit comprising the diagnostic composition, a method for providing information for diagnosis of a neurodegenerative disease, cancer, or a viral infection disease which are in pathological correlation with G-quadruplex, and a pharmaceutical composition for prevention or treatment of a neurodegenerative disease, cancer, or a viral infection disease, the pharmaceutical composition comprising JULGI protein, a protein having a homology of 80% or more with the JULGI protein, or a mutant protein thereof. Thanks to the activity of binding to the G-quadruplex structure, JULGI according to the present disclosure is expected to be advantageously used in diagnosing or treating various diseases that are in pathological correlation with the abnormal formation and dissociation of the G-quadruplex structure.