Molecular target anticancer drugs have less adverse effects than conventional compounds but often provide patients with unfavorable events. Development of an appropriate administration protocol that alleviate the adverse effects and maximize the desirable effects requires monitoring of drug concentrations in individuals. This strategy is currently inaccessible, owing to no convenient method usable at a clinical site. Furthermore, as for recent drugs whose therapeutic windows remain unevaluated, it is difficult to control the adverse effects. To address these shortcomings, we describe a rapid and simple procedure for determination of the concentrations in blood samples using lenvatinib, a multi-kinase inhibitor, as a test reagent. This method stems from electrochemical measurement with diamond sensor that induces more stable reaction than classical materials. When guinea-pig plasma mixed with lenvatinib was examined, the sensor could detect a clinically relevant concentration of ≥ 1 µM. Time necessary for all the processes including sample's pretreatment did not exceed 10 minutes. This procedure may contribute to advances in personalized medicine.