The properties of individual cells are thought to depend on the time course change of the number and the spatial distribution of regulatory molecules in cells. To understand the strict meaning of those roles of some particular sets of regulatory proteins, we need to measure both the phenotypical characteristics of cells and the quantitative amount of mRNAs in each of those individual cells. For that purpose, we have developed a novel method to measure the quantitative amount of mRNA expression in individual cells keeping their spatial distributions in the cell without any amplification process like PCR. In this method, a set of different sizes of gold nano-particles attached with different probe-DNA respectively were used as a set of probes to detect different mRNAs existing in a cell. At first, the optimum condition of the immobilization of probe-DNA onto the gold nano-particle surface was examined. Next, the selectivity of the probe-DNA immobilized onto the nano-particle was tested using complementary oligonucleotide molecules. We confirmed the several different kinds of gold nano-particle probes were hybridized with target oligonucleotides having complementary sequences with almost 100% selectively. For the counting and distinguishing each of the gold nano-particles, we used two different methods and compared: one is Atomic Force Microscopy and the other is Scanning Electron Microscopy. Quantitative detection of mRNAs in individual cells keeping their spatial distributions was then examined using the gold nano-particle-based detection system. In this meeting, we will present the results and will discuss about the potential and problems of this method for the single-cell-based quantitative expression analysis.