Compost is widely used in tobacco cultivation in Japan, and the application of good compost produces good quality tobacco leaves. It is an effective way of knowing the importance of compost to understand its (micro) biological effects in addition to its chemical and physical effects as well as its quality. Thus a study was conducted as to how microbial counts, especially that of plant growth-promoting rhizobacter (PGPR), in the soil and root areas changed as a results of compost application and according to its type. Three types of compost were used: straw, cow manure and chicken manure. The total number of bacteria, gram-negative bacteria (GN), fluorescent pseudomonads (FP) and fungi in the soil samples to which the compost was applied and in the control soil was counted by the dilution plate technique. As a result, FP, a PGPR, was found less in the chicken manure soil than in the control soil. Then the effects of three types of compost on the microbial counts in the rhizoplane were studied using spinach. After the aseptically germinated seeds of spinach were sown in the soil samples, the plants was cultured in a growth chamber for three weeks and then the above-mentioned microorganisms in the samples were examined. It was found that FP and GN existed more in the cow manure soil samples than in the control soil. As noted, the microbial counts in the soil and in the rhizoplane differed according to the compost type, which suggested that compost affected microbial counts. In addition, many FP were found in the cow manure compost itself. This indicated the possibility that the FP from compost propagated in the rhizoplane, too. The identification of the FP isolated from the spinach plant's rhizoplane showed that almost all of them were Pseudomonas fluorescens biovar II or IV.