PURPOSE: To enable a coal expans property index to be measured regrdless of a piston load and a pencil length (coal filling layer thickness) which is suited for absolute evaluation of a coal viscosity.

CONSTITUTION: After a finely crashed coal sample is charged into a prescribed capillary in a particle with a specified charging density, heating is made at a prescribed heat-up speed by applying a load onto it or without applying it by a piston. An expansion property of the sample is expressed by a ratio V/W (specific volume: unit [cm3/dry, g-coal]) between a specific volume (V) of coal after expansion and a coal weight (W) at a dry coal base.