There are provided a cluster of thin sheet graphite crystals or the like which is useful as an electrode material for lithium ion batteries, hybrid capacitors and the like, and a method for efficiently producing the same at high productivity. The method is one for producing a cluster of thin sheet graphite crystals composed of aggregates in such a state that thin sheet graphite crystals extend from the inside toward the outside, comprising charging a powdery and/or particulate material of an organic compound pre-baked to an extent of containing remaining hydrogen in a graphite vessel, and subjecting the powdery and/or particulate material together with the vessel to hot isostatic pressing treatment (HIP treatment) using a compressed gas atmosphere under the predetermined conditions.