We show an experimental technique for visualizing distributions of vibrational modes of molecules through mapping of an inelastic tunneling signal with a scanning tunneling microscope. A topographic information and d 2 I/d V2 signal processed by a lock-in amplifier were simultaneously imaged, where the feedback loop for the tunneling gap was engaged and a modulation voltage was superimposed to the gap voltage. The current signal used for the tunneling gap control was tuned by the filtering in order to minimize the response of the feedback loop caused by the modulation voltage. The effectiveness of this technique was demonstrated for a self-assembled monolayer composed of a mixture of normal and deuterated hexanethiol molecules, where both molecules have the same molecular length and the former was embedded in the matrix of the latter. Two types of molecules were successfully discriminated by chemical properties.