Thin film gas sensor based on tin oxide was fabricated and its characteristics were examined. Target gas is acetonitrile ($CH_{3}CN$) that is simulant gas of blood agent gas. Sensing materials are $SnO_{2}$, $SnO_{2}$/Pt, and (Sn/Pt)oxidation with thickness from $1000{AA}$ to $3000{AA}$. Sensor was consisted of sensing electrode with interdigit (IDT) type in front side and a heater in back side. Its dimension was $7{ imes}10{ imes}0.6mm^{3}$. Fabricated sensor was measured as flow type and monitored real time using PC. The optimal sensing material for $CH_{3}CN$ was {Sn($3000{AA}$)/Pt($30{AA}$)}oxidation and its sensitivity and operating temperature were 30%, $300^{circ}C$ in $CH_{3}CN$ 3 ppm.