To enable easy implementation of three-dimensional (3D) characterization of flowing objects, we have developed a mirror-embedded microchannel to obtain 3D positional information from two different orthogonal-axis images. A silicon mirror reflects the side view of the channel and enables simultaneous imaging of the top and side views of the channel with a single lens. The measurement principle was verified by observing fluorescent streams simultaneously in both horizontal and vertical directions of a microchannel and measuring 3D positions of 6 and 10-mu m sized beads without any optical modification of a microscope and additional optical equipment.