A multiplexer for controlling a fluid in a microchannel by controlling pneumatic pressure in the microchannel in a microfluidics chip includes: a first pneumatic channel; and a second pneumatic channel forming a cross point which is in communication with the first pneumatic channel, wherein the cross point is in communication with the microchannel of the microfluidics chip, and predetermined pneumatic pressure is provided to the microchannel by using a combination of providing of the pneumatic pressure to the first and second pneumatic channels, channel closing, or channel opening.