PURPOSE: To make a diagnosis always in a combustion state by measuring contour and shape of char bed, calculating a geometrically and specifically very small amount thereof and evaluating and judging from degree of similarity to a standard geometrically and specifically very small amount.

CONSTITUTION: In evolving steam by burning black liquor produced in a pulp production process is burnt in a char bed, contour and shape of char bed to be a standard in combustion state is previously measured to calculate a geometrically and specifically very small amount to show the contour and shape of the standard geometrically and specifically very small amount. Then contour and shape of char bed in operation is measured to obtain the above-mentioned geometrically and specifically very small amount, single similarity and complex similarity of both the geometrical amounts. When the complex similarity is small than a give evaluated and judged value, the combustion is judged in an unstable state and an alarm is raised to diagnose combustion of recovery boiler.