We propose a miniature piezoelectric mixer using a bending vibration mode of a metallic square cuboid vibrator. The shape of the mixer is a square cuboid (14 × 14 × 10 mm) with a through-hole of 7 mm in diameter at its center. When AC voltages are applied to the piezoelectric elements bonded to the sides of the mixer and the bending vibration mode is excited, the water in the through-hole generates a rotational flow. One of the advantages of the mixer is simplicity of the structure and it makes the mixer miniaturized. In this report, we build the prototype of the mixer and verify the driving principle of the flow generation by finite element method (FEM) analysis and computational fluid dynamics (CFD) analysis. The mixing performance such as a flow velocity is shown in the experiment. We build another miniature mixer, the shape of which is a cube with side length of 5 mm with a through-hole of 3 mm and demonstrate the flow generation.