Methadone is a difficult medicine to assess the efficacy at an initial stage because the blood concentration of it varies greatly among individuals and it takes days to reach a steady state and cannot be increased for 7 days. Nevertheless, there are few reports of blood concentration together with effects after administration of methadone about Japanese cancer patients. In this study, we investigated changes in blood concentration and pain score (NRS), and factors that affect blood concentration. Dose per body weight was only correlated with blood concentration of methadone. In the effective cases, NRS decreased chronologically until the 7th day after treatment initiation, and significantly decreased from the 1st day compared to before treatment initiation, but in the ineffective cases, it tended to decrease until the 3rd day, but there was no change thereafter. The blood concentration increased to 110 ng/ml on the 7th day in the effective cases, and in the ineffective cases, it reached the concentration on the 3rd day. Thus there was no correlation between the blood concentration and the drug efficacy. The individual blood concentrations tended to increase slightly or decrease after the 3rd day, but in only one case, it continued to increase. From the above-mentioned, it was shown that the effect could be judged at an early stage, however, since there was a case in which the blood concentration continued to rise until the 7th day, it was considered that the early dose increase within 7 days after initiation should be performed carefully.