PURPOSE: To easily grasp various factors for deforming a substrate by disposing a plurality of light sources for projecting light onto the surface of semiconductor substrate placed in a heat treatment furnace and then moving the light sources, semiconductor substrates, and a surface imaging unit while interlocking without varying relative positions.

CONSTITUTION: A silicon tube 1 is placed in a heat treatment furnace and a silicon board 2 mounting silicon wafers 3 is placed on an SiC fork 22. Three fluorescent lamps and a lattice of predetermined interval 10 are disposed at a position where the fork 22 is secured while a mirror 11 and an imaging video camera 12 are secured thereto so that relative position between the fork 22, the wafer 33, the fluorescent lamp 9, the lattice 10, the mirror 11, and the camera 12 does not vary. The camera 12 simply image-pickups a square lattice 10 so long as the wafer 3 is intact. Upon deformation of the wafer 3, light of the fluorescent lamp passes through the lattice 10 and reflects on the surface of the wafer 3 to be imaged on the mirror 11 thus providing distribution of lattice image distortion in the wafer surface corresponding to deformation at each part thereof.