PURPOSE: To activate a liquid fuel by bringing it into contact under agitation with a granular solid comprising an energy radiator.

CONSTITUTION: A container 1 is a stainless steel cylinder with its top covered with a lid 2. The container 1 has an outlet pipe 4 for an activated fuel on its bottom, and a screen is set on the upper end of the pipe 4. An inlet pipe 3 for fuel penetrates through the lid 2 and extends to the vicinity of the bottom wall 1b. A liquid fuel 7 (e.g. heavy oil) injected into the container 1 is subjected to the so-called activation by contact under agitation with a granular solid 6 (ceramic granules which can radiate far-infrared energy) and discharged gradually from the outlet pipe 4. This treatment can improve the flow and combustibility of the fuel and heighten its heating value. Examples of the solids include an infrared radiator of a wavelength peak of 3-15μm, a single or compound energy radiator which radiates α, β or γ-rays of an intensity of 1000CMP or above, and a magnetic substance which radiates a magnetic flux of a flux density of 300G or above.