A CO gas sensor which works at room temperature without an inner heater has been investigated. The sensing material is ceramics made from hydrous SnO₂ gel with Pt dispersion. The effect of calcination in the range from room temperature to 1100°C was studied. The increment of crystallite size of SnO₂ started from the calcination temperature of 400°C, and increment of its size was lowered by Pt dispersion. The crystallite size of Pt increased in the range from 650°C to 800°C and leveled off above 800°C. The specific surface area of Pt-dispersed hydrous SnO₂ gel was larger than that of hydrous SnO₂ gel without Pt. The relation between current (I) and voltage (V) was found to be I = KVα. The value of α was larger than 1 for calcination temperature below 650°C for Pt-dispersed hydrous SnO₂ gel, and at 450°C the value of α showed a maximum. The conductance of Pt-dispersed hydrous SnO₂ gel was less than that of hydrous SnO₂ gel without Pt. The ceramic with Pt/Sn = 4.4 mole% calcined at 450°C gave the best response to CO gas.