We have developed a wavelength-swept laser source with ultrahigh phase stability. A potassium tantalate niobate (KTa1-xNbxO3, KTN) single crystal was employed as an electro-optic deflector for a high-speed wavelength sweep in the laser cavity. The device structure and performance of a KTN deflector is described. The device includes the beam-shaping optics, which can enhance the resolution of a KTN deflector. A 200-kHz sweep rate was obtained with an average output power of 20mW and a coherence length of 8mm for a wavelength range exceeding 100nm. We demonstrated a swept source with ultrahigh phase stability in the 1.3µm wavelength range as a result of the low-jitter operation of the deflector. The standard deviation of timing jitters measured between adjacent A-lines was confirmed to be less than 78ps, which corresponds to a phase difference of 0.017 radians at a Michelson interferometer path difference of 1.5mm. In addition to realizing the phase stability of neighboring A-lines, a long-term stable sweep was demonstrated by eliminating the refresh operation that was previously needed to prevent output power decay.