PURPOSE: To further improve the homogeneity of temperature in the plane of a gel.

CONSTITUTION: Spacers 42 are pinched at both side sections of two transparent glasses 40a, 40b to form a gap between the glass plates 40a, 40b, and a gel 2 is formed in the gap. Soaking plates 46a, 46b are arranged on the outer surfaces of the glass plates 40a, 40b via radiating sheets 44a, 44b, and heat insulating materials 48 are provided at the side sections of the glass plates 40a, 40b, radiating sheets 44a, 44b, and soaking plates 46a, 46b. The heat radiation from both sides of the gel 2 is prevented by the heat insulating materials 48, and the homogeneity of temperature in the plane of the gel 2 is improved.