According to one embodiment, a biometric authentication device includes a resin substrate, an optical sensor and an illumination device. The resin substrate has flexibility. The optical sensor is disposed on the resin substrate. The illumination device is disposed on the resin substrate. The optical sensor and the illumination device are disposed on the resin substrate so as to face each other with a detection target interposed therebetween when the biometric authentication device is mounted on the detection target. The optical sensor detects light emitted from the illumination device and transmitted through the detection target.