An estimation device is applicable to a combustion system including an internal combustion engine. The estimation device includes a mixing acquisition unit, a combustion amount estimation unit, and a region estimation unit. The mixing acquisition unit acquires the mixing ratio of various components contained in the fuel used for combustion in the internal combustion engine. The combustion amount estimation unit estimates a main combustion amount of the fuel caused by a main combustion produced by injecting the fuel into a combustion chamber of the internal combustion engine with a main injection, based on the mixing ratio acquired by the mixing acquisition unit. The region estimation unit estimates a combustion region of the main combustion in the combustion chamber based on the mixing ratio acquired by the mixing acquisition unit.