Abstract: A checking paper for use in a sheet combustion type oxygen concentration measuring instrument, comprising a thin sheet of paper bonded to plate-like holder made of cardboard, plastic or metal plate, said holder having a cutout portion of a suitable configuration such as an acute-angle isosceles triangle extending from one end of the holder to the other so that said paper sheet is partly exposed through said cutout portion of the holder. The base side end of the exposed portion of said paper sheet is extended beyond the open end of the holder and an igniting agent is provided to such extended portion, or the opening of the open end of said holder is widened to facilitate combustion and its propagation.