A passive sampler has been developed for measuring formaldehyde in air using methyl yellow and methyl orange indicator. It is based on the color change of the TLC plate on reaction with formaldehyde. TLC plates impregnated with three kinds of solutions containing methyl orange, methyl yellow, mixing of methyl yellow and methyl orange were shown rectilinear response to formaldehyde concentration at constant exposure time. The color change of impregnated TLC plate with criteria air pollution, such as NO$_2$, SO$_2$, CO, $O_3$ was investigated. The color change of TLC plate with $O_3$ is larger than other gases.