We report a high-sensitivity fiber-optic ultrasonic sensor system using a self-injection-locked distributed-feedback (DFB) diode laser where a π-phase-shifted fiber Bragg grating (πFBG) serves as both the locking resonator and the sensing element in a fiber ring feedback loop. By controlling the delay time of the feedback light through a fiber stretcher, the laser wavelength is locked to an external cavity mode on the spectral slope of the πFBG, and the ultrasound-induced wavelength shifts of the πFBG are converted to laser intensity variation. The ultrasonic sensing scheme simplifies the feedback control because the self-injection locking automatically pulls the laser wavelength to the πFBG resonant wavelength. In addition, it improves the detection sensitivity because the frequency noise of the DFB laser is drastically reduced. We show that the sensor system achieves a strain sensitivity of 78 fε/Hz1/2 at around 200 kHz.