PURPOSE: To enable accurately measuring operation gas temperature by providing a grid downstream side of final step of the step part with a pair of stationary blade and bucket and a camera sensing the radiation of the passing operation gas.

CONSTITUTION: Among the steps of a gas turbine, the pair of stationary blade 12 and bucket 13 at the final step is defined as the step LS. The step LS is covered with a casing 11 extending along the gas turbine axis and a bearing support 14 extending to radial direction is provided downstream side. On the way to the support 14, provided are an infrared camera 16 and a grid 15 of lattice shape perpendicularly to the gas flow. The operation gas having flown out of the step LS exhibits radiation heat when passing through the grid 15, which is photographed with the camera 16 and image-processed to grasp the temperature distribution of the gas. This temperature distribution is compared with the distribution in normal operation and if there is an anomaly, the existence of abnormal incineration in a specific can be estimated from the position and the degree.