A heat-curable resin composition for semiconductor encapsulation, comprising: a cyanate ester compound having not less than two cyanato groups in one molecule, the cyanate ester compound containing a cyanate ester compound (A-1) represented by the following formula, the cyanate ester compound (A-1) being contained in the whole cyanate ester compound as the component by an amount of not smaller than 90% by mass, and exhibiting a viscosity of not higher than 50 Pa·s at 23°C when measured by a B-type rotary viscometer in accordance with a method described in JIS K7117-1:1999 wherein n represents an integer of 0 or 1; each of R 1 and R 2 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms; and R 3 represents a divalent linking group selected from the groups expressed by the following formulae to -CH2- -O-; a phenol curing agent containing a resorcinol-type phenolic resin represented by the following formula, the component being contained at a ratio at which the cyanato groups in the component is in an amount of 0.5 to 100 equivalents per 1 equivalent of the hydroxyl groups (OH groups) in the component, wherein n represents an integer of 0 to 10; each of R 1 and R 2 independently represents a hydrogen atom or a monovalent group selected from the group consisting of an alkyl group having 1 to 10 carbon atoms, an allyl group and a vinyl group; and x represents 1 or 2; a curing accelerator in an amount of 0.01 to 5 parts by mass per 100 parts by mass of the component; an inorganic filler comprising particles having either spherical or deformed sphere shapes, exhibiting on average an aspect ratio of 1 to 4, has an average particle diameter of 1 to 20 μm when measured by a laser diffraction method, the component being in an amount of 1,200 to 2,200 parts by mass per 100 parts by mass of a sum total of the components and, and has been surface-treated with a silane coupling agent represented by the following formula, the silane coupling agent being contained in an amount of 0.2 to 0.5 parts by mass per 100 parts by mass of the inorganic filler, R 1 a (OR 2) (3-a) Si-C 3 H 6 -R 3 wherein a represents an integer of 0 to 3; R 1 represents a methyl group or an ethyl group; R 2 represents an alkyl group having 1 to 3 carbon atoms; and R 3 represents a group selected from the group consisting of the nitrogen-containing functional groups represented by the following formulae to -NH 2 -NHC 6 H 5 -NHC 2 H 4 NH 2 and an ester compound that is in an amount of 1 to 10 parts by mass per 100 parts by mass of the sum total of the components and, and is represented by the following formula R 4 -CH 2 CH 2 -NH-CH 2 CH 2 -R 5 wherein each of R 4 and R 5 represents a saturated ester residue having 2 to 30 carbon atoms. a cyanate ester compound having not less than two cyanato groups in one molecule, the cyanate ester compound containing a cyanate ester compound (A-1) represented by the following formula, the cyanate ester compound (A-1) being contained in the whole cyanate ester compound as the component by an amount of not smaller than 90% by mass, and exhibiting a viscosity of not higher than 50 Pa·s at 23°C when measured by a B-type rotary viscometer in accordance with a method described in JIS K7117-1:1999 wherein n represents an integer of 0 or 1; each of R 1 and R 2 represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms; and R 3 represents a divalent linking group selected from the groups expressed by the following formulae to -CH2- -O-; a phenol curing agent containing a resorcinol-type phenolic resin represented by the following formula, the component being contained at a ratio at which the cyanato groups in the component is in an amount of 0.5 to 100 equivalents per 1 equivalent of the hydroxyl groups (OH groups) in the component, wherein n represents an integer of 0 to 10; each of R 1 and R 2 independently represents a hydrogen atom or a monovalent group selected from the group consisting of an alkyl group having 1 to 10 carbon atoms, an allyl group and a vinyl group; and x represents 1 or 2; a curing accelerator in an amount of 0.01 to 5 parts by mass per 100 parts by mass of the component; an inorganic filler comprising particles having either spherical or deformed sphere shapes, exhibiting on average an aspect ratio of 1 to 4, has an average particle diameter of 1 to 20 μm when measured by a laser diffraction method, the component being in an amount of 1,200 to 2,200 parts by mass per 100 parts by mass of a sum total of the components and, and has been surface-treated with a silane coupling agent represented by the following formula, the silane coupling agent being contained in an amount of 0.2 to 0.5 parts by mass per 100 parts by mass of the inorganic filler, R 1 a (OR 2) (3-a) Si-C 3 H 6 -R 3 wherein a represents an integer of 0 to 3; R 1 represents a methyl group or an ethyl group; R 2 represents an alkyl group having 1 to 3 carbon atoms; and R 3 represents a group selected from the group consisting of the nitrogen-containing functional groups represented by the following formulae to -NH 2 -NHC 6 H 5 -NHC 2 H 4 NH 2 and an ester compound that is in an amount of 1 to 10 parts by mass per 100 parts by mass of the sum total of the components and, and is represented by the following formula R 4 -CH 2 CH 2 -NH-CH 2 CH 2 -R 5 wherein each of R 4 and R 5 represents a saturated ester residue having 2 to 30 carbon atoms.