A new technical means is provided that can be produced and prepared at low cost, and in which the generation of stagnation in the flow of a liquid sample can be suppressed, microbeads can be smoothly and simply injected, filled and transported and the generation of dust can be also suppressed and a new extracting method for nucleic acid using the new technical means is provided. A microchip has a microchannel (3) formed by groove parts (11, 21) provided in connecting surfaces of upper and lower substrates (1, 2). In the microchannel (3), a gap part (31) in which the section of the channel is reduced is provided in a central part upward and downward, rightward and leftward, or upward and downward and rightward and leftward the section thereof, and microbeads are checked.