  We investigate the phase structure of three-flavor QCD in the presence of
finite quark chemical potential $\mu/T\lesssim1.2$ by using the
non-perturbatively $O(a)$ improved Wilson fermion action on lattices with a
fixed temporal extent $N_{\rm t}=6$ and varied spatial linear extents $N_{\rm
s}=8,10,12$. Especially, we focus on locating the critical end point that
characterizes the phase structure, and extracting the curvature of the critical
line on the $\mu$-$m_{\pi}$ plane. For Wilson-type fermions, the correspondence
between bare parameters and physical parameters is indirect. Hence we present a
strategy to transfer the bare parameter phase structure to the physical one, in
order to obtain the curvature. Our conclusion is that the curvature is
positive. This implies that, if one starts from a quark mass in the region of
crossover at zero chemical potential, one would encounter a first-order phase
transition when one raises the chemical potential.
