A liquid crystal chopper (4) comprising a pair of electrically conductive substrates (4b,c), a spacer (4d) keeping the substrates so as to define a space between the substrates, and a liquid crystal (4a) of DSM type which is enclosed in the space, with a dielectric anisotropy of -0.5 or more and not exceeding 0. An infrared sensor using the liquid crystal chopper(4) in the above construction as a chopper mechanism to intermittently interrupt infrared rays incident upon the pyroelectric element (2). The infrared sensor has a short response time at the liquid crystal chopper and consumes less power.