An estimation device is applicable to a combustion system including an internal combustion engine and includes a mixing acquisition unit, a main region estimation unit, and an after region 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 main region estimation unit estimates a combustion region of the fuel as a main combustion region for a main combustion produced by injecting the fuel into a combustion chamber of the internal combustion engine by main injection, based on the mixing ratio acquired by the mixing acquisition unit. The after region estimation unit estimates an injection region of the fuel as the after combustion region based on the mixing ratio, for an after combustion produced by injecting the fuel into the combustion chamber by an after injection, after the main injection in one combustion cycle.