In this study, we proposed a method of measuring respiration using a smart seatbelt buckle (SSB) as a method of measuring biometric signals without a load of attachment. The accuracy of the calculated breath time was evaluated, then we attempted to estimate the drowsiness level using the breath time while driving a simulator. As a result, high correlation was obtained between breath times by SSB and band-type thoracoventral respiration sensors measured as a reference. And the possibility to estimate drowsiness level 4 or more was indicated.