We have investigated an insertion process of conjugated molecules, 4-biphenylmethanethiol (BP) and [1,1':4',1"-terphenyl]-4-methanethiol (TP) into preassembled n-alkanethiol self-assembled monolayers on Au(111) using scanning tunneling microscopy (STM), Ex situ STM observation revealed that the insertion process is dependent on an organic solvent in which the conjugated molecules dissolve. The insertion of conjugated molecules with three phenyl rings (TP) was faster than that with two phenyl rings (BP). By analyzing the STM images, the lower limits of resistances for single BP and TP molecules were obtained as 22+/-5 and 36+/-5 G Omega, respectively.