The purpose of this thesis is to explore other yet undiscovered contrast origins and unused contrast agents of third-harmonic-generation (THG) inside biological tissues, and apply to the preliminary studies of medicine based on a nonlinear laser scanning microscopy system using a femtosecond Cr: forsterite laser centered at 1,230-nm. The scope of this thesis contains that THG contrast from elastic fibers is detected and demonstrated, and using acetic acid or hematoxylin as THG contrast agents is verified to be feasible.