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We propose a new method to optimize the core alignment of the butt coupling between a single-mode optical fiber (SMF) and a graded-index multimode optical fiber (GI-MMF) by monitoring the spectrum of Brillouin scattered light. This method is stable, free from costly or elaborate equipments, applicable to the coupling of optical fibers with different core diameters or materials, and available even when the power of transmitted light cannot be measured. We experimentally show the usefulness of this method in the following three cases: where a silica-based SMF is butt-coupled to 1) a silica-based GI-MMF with 50-μm core diameter; 2) a perfluorinated GI polymer optical fiber (PFGI-POF) with 62.5-μm core diameter; and 3) a PFGI-POF with 120-μm core diameter. Depending on the relative core position, not only the Stokes power but also the Brillouin frequency shift changed, which probably originates from the excitation of higher order modes.