We have compared the column densities of H 2 C O , 13 C O , C 18 O , 12 C O , CS, and H C O + to the visual extinctions derived from star counts in eight dark clouds, L1317, B1, L1551, L1535, L1544, L134, L134N, and B335. We examined the degree of correlation between molecules and extinctions. The contours of 13 C O , H 2 C O , and CS distributions show an excellent coincidence with those of the visual extinction. Even though the plots of molecule brightness temperature or column density as a function of the visual extinction show a considerable scatter, a good correlation is found between the two quantities. The turnover of the ratios of 13 C O and H 2 C O column densities to the visual extinction at high extinctions observed in several clouds is not apparent in this work. The ratios seem to maintain constant values within the range of A V we studied. The slopes of the relations between the molecule column density and extinction, and the threshold values of extinction for detection of molecules are comparable to the other works. The cause of the slope difference is discussed. We derived, for the first time, the relations of CS and H C O + column densities and the visual extinction. The ratio of CS column density to extinction is at least two orders of magnitude lower than the mean value for 13 C O , but the threshold for detection of CS is comparable to that of 13 C O . The ratio of the H C O + column density to extinction is one to two orders of magnitude lower than the mean ratio for H 2 C O .