Analysis of environmentally sensitive materials is essential for developing and optimizing nanostructured photochemical materials and devices. Photoinduced force microscopy (PiFM) is a promising local spectroscopic technique to visualize nanoscale local optical responses by measuring the optical forces between the scanning tip and sample. In this study, we examined isolated single quantum dots (QDs) with ligands on a gold substrate via PiFM under ultra-high vacuum to characterize the QD adsorption state on the basis of the optical force. The strong self-consistent optical interaction through the tip-substrate plasmonic gap induced by laser light modulates the PiFM image depending on QD crystal existence in the gap. This observation clarified the QD absorption situation on the substrate, and the crystal position in the QDs was determined even though the ligand walls covered the crystal. This insight concerning force spectroscopy can aid further research on the photochemistry of nanostructured materials and molecular spectroscopy.