Energy transfer is an essential dynamic process in naturally configured photosynthetic systems, and it has also been used in various artificially designed energy-harvesting devices. Diverse functions have been realized by different regulations of the energy dynamics, and the microscopic understanding and molecularlevel control of the energy transfer are long-standing challenging targets. So far, optical spectroscopy has been used to investigate excitation dynamics, however, the spatial resolution of conventional optical spectroscopy is limited, and a large part of energy transfers on the nanoscale is still unknown. Here I introduce our recent progress in measurement of the single molecule luminescence and absorption spectra of a single molecule using a scanning tunnelling microscope (STM) equipped with optical detection facilities. With the spectroscopic techniques, we conducted a molecular-level investigation of energy transfers in molecular dimers consisting of a free-base phthalocyanine and magnesium phthalocyanine (H2Pc and MgPc) by absorption/emission spectroscopy using STM (Fig. 1a) [1,2]. As shown in Fig. 1b, a luminescence signal from H2Pc at 1.81 eV was detected while locally exciting a nearby MgPc with the tunneling current of STM, clearly indicating an energy transfer from MgPc to H2Pc. The mechanism of the energy transfer is proven to be resonance energy transfer (RET) because charge transfer is prohibited by the energy-level alignment at the MgPc-H2Pc heterojunction.