PROBLEM TO BE SOLVED: To provide a formic acid decomposing device and a decomposing method of formic acid capable of safely and quickly treating formic acid and to provide a catalyst activity diagnostic method for determining the activity of a catalyst filled in the formic acid decomposing device.SOLUTION: In a decomposing method of formic acid, gas containing formic acid and oxygen or gas containing oxygen are reacted with each other in the presence of a catalyst for decomposing formic acid, and formic acid is decomposed into water and carbon dioxide. A device for decomposing formic acid which decomposes formic acid into water and carbon dioxide includes a formic acid decomposition part in which the catalyst for decomposing formic acid is filled, a gas introduction pipe through which gas containing formic acid is introduced into a formic acid decomposition part and a gas introduction mechanism through which oxygen or gas containing oxygen is introduced into the formic acid decomposition part. In a catalyst activity diagnostic method, gas containing formic acid, and oxygen or gas containing oxygen are introduced into the formic acid decomposition part, a change of catalyst temperature due to heat generation upon decomposition reaction of formic acid is measured by a thermo-couple disposed on a catalyst layer of the formic acid decomposition part and the activity of the catalyst is determined from the extent of an amount of change of elevated catalyst temperature.
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