To provide a thermal decomposition method capable of thermally decomposing tar sand underground directly, without separating heavy oil from the tar sand nor transferring to the ground and requiring no construction of expensive plants such as a thermal decomposition system on the ground, and to provide a thermal decomposition system for the above method.

The thermal decomposition method comprises forming underground partitions from the earth surface to the bedrock in a stratum where the tar sand is buried so as to separate a part of a tar sand layer from other areas and to provide an underground reaction vessel, and supplying heat energy to the underground reaction vessel to perform thermal decomposition reaction of the tar sand, and the thermal decomposition system of the tar sand is also provided.