We propose an ultrahigh quality factor (Q) photonic crystal slab cavity created by the local modulation of the air hole radius in the waveguide. In the cavity, photons are confined between two mirror regions with larger air holes and the lifetime of photons is greatly enhanced by introducing tapered regions with linearly increasing air hole radii. Q and mode volume are investigated for the cavities with various lengths and air hole size offsets of the tapered region with linearly increasing air hole radii by three-dimensional finite-difference time-domain method. The behaviors are analyzed by the mode patterns in real space and wavevector space. We obtain a numerical Q up to 8.8 x 10(7) for a mode volume of 1.6 (lambda/n)(3). Concerning the waveguide coupling, the cavity shows 80% coupling efficiency while keeping Q higher than 10(6).