Negative refraction of acoustic waves in a two-dimensional (2D) phononic crystal is studied by numerical simulation based on the finite-difference time-domain (FDTD) method. We calculate the phonon band structure of 2D phononic crystals, consisting of metal cylinders placed periodically in a liquid. By comparing several combinations of materials for metal cylinder and liquid, we analyze the dependence of the band structures on sound speed and density of liquid media. The negative refraction of the acoustic waves is observed at the interfaces between the phononic crystal slab and the liquid. We find that an acoustic "lens effect" with the slab appears due to the negative refractions. The relationship between the focal intensity in the lens effect and the band structure of the phononic crystal is quantitatively discussed. [doi: 10.2320/matertrans.MC200823]