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