Although valuable microbes have been isolated from the soil for the various productions of useful components, the microbes which can be cultivated in the laboratory are only $0.1-1\%$ of all microbes. To solve this problem, the study has recently been tried for making the valuable components from the environment by directly separating unculturable micrbial DNA in the soil. But it is known that humic acid originated from the soil interrupts various restriction enzymes and molecular biological process. Thus, in order to prevent these problems, this study modified the method separated soil DNA with phenol, CTAB and PEG. In order to compare the degree of purity for each DNA and the molecular biological application process, $A_{260}/A_{280}$ ratio, restriction enzymes, and PCR were performed. In case of DNA by the modified method, total yield of DNA was lower but $A_{260}/A_{280}$ ratio was higher than the previously reported methods. It was confirmed that the degree of purity is improved by the modified method. But it was not cut off by all kinds of tested restriction enzymes because of the operation of a very small amount of interrupting substances. When PCR was operated with each diluted DNA in different concentrations and GAPDH primer, the DNA by the modified method could be processed for PCR in the concentration of 100 times higher than by the previously reported separation method. Therefore, this experiment can find out the possibility of utilization for the unknown substances by effectively removing the harmful materials including humic acid and help establishing metagenomic DNA library from the soil DNA having the high degree of purity.