PURPOSE: To perform a uniform heating to a rod-like sample by a method wherein a light-emitting amount distribution in each of minute line segment is corrected so that multi-degree function of even number having a multiple root may be set in the center of the line segment in the longitudinal direction.

CONSTITUTION: A linear light source 1 is placed at one focal point of an ellipse mirror surface 3, a rod-like sample 2 is placed on the other focal point, wherein light outputted from the linear light source 1 is collected at a focal point where the rod-like sample 2 is located so as to heat the rod-like sample 2. When the rod-like sample 2 is heated, the linear light source 1 is provided with means for correcting a distribution of emitting light amount of each of the minute linear components in such a manner that a multi-degree function of even number of degree with a multiple root at a central linear part to delete a variation of the received light amount on a y-axis of the rod-like sample 2 placed at one focal point. With such an arrangement as above, it is possible to get a uniform heating against the rod-like sample 2.