A heat-curable resin composition comprising: a heat-curable resin mixture containing a heat-curable resin (a) formed of an epoxy resin and thickener particles (b) dispersed in the heat-curable resin (a) and having a content of 5 to 40 parts by mass with respect to 100 parts by mass of the heat-curable resin (a), the heat-curable resin mixture exhibiting a viscosity S of from 10 to 300 Pa·s at a temperature of 150°C, obtained after the heat-curable resin mixture is held at a temperature of 150°C for 30 seconds after the temperature was raised to 150°C at a temperature rising rate of 40°C/min at an angular velocity of 10 rad/sec, and a curing agent having a content of 5 to 30 parts by mass with respect to 100 parts by mass of the heat-curable resin (a), wherein the heat-curable resin composition exhibits a lowest viscosity R at a temperature of 80 to 120°C of from 0.1 to 10 Pa·s, and the lowest viscosity R is measured using a temperature-viscosity curve obtained from viscosity measurement up to 180°C at a temperature rising rate of 2°C/min at an angular velocity of 10 rad/sec, wherein viscosity R and viscosity S are determined according to the description, and the viscosity S and the lowest viscosity R satisfy the following formula 5 < S / R < 200 wherein the thickener particles (b) are formed of a polymer constituted by one or more polymerization units selected from the group consisting of a methacrylate compound, an acrylate compound, and a vinyl compound, and the thickener particles (b) are capable of increasing the viscosity of the heat-curable resin composition by 10 times or more, measured at 150°C after the heat-curable mixture is held at 150°C for 30 seconds, when the thickener particles (b) are contained in the composition with 10% by mass. a heat-curable resin mixture containing a heat-curable resin (a) formed of an epoxy resin and thickener particles (b) dispersed in the heat-curable resin (a) and having a content of 5 to 40 parts by mass with respect to 100 parts by mass of the heat-curable resin (a), the heat-curable resin mixture exhibiting a viscosity S of from 10 to 300 Pa·s at a temperature of 150°C, obtained after the heat-curable resin mixture is held at a temperature of 150°C for 30 seconds after the temperature was raised to 150°C at a temperature rising rate of 40°C/min at an angular velocity of 10 rad/sec, and a curing agent having a content of 5 to 30 parts by mass with respect to 100 parts by mass of the heat-curable resin (a), wherein the heat-curable resin composition exhibits a lowest viscosity R at a temperature of 80 to 120°C of from 0.1 to 10 Pa·s, and the lowest viscosity R is measured using a temperature-viscosity curve obtained from viscosity measurement up to 180°C at a temperature rising rate of 2°C/min at an angular velocity of 10 rad/sec, wherein viscosity R and viscosity S are determined according to the description, and the viscosity S and the lowest viscosity R satisfy the following formula 5 < S / R < 200 wherein the thickener particles (b) are formed of a polymer constituted by one or more polymerization units selected from the group consisting of a methacrylate compound, an acrylate compound, and a vinyl compound, and the thickener particles (b) are capable of increasing the viscosity of the heat-curable resin composition by 10 times or more, measured at 150°C after the heat-curable mixture is held at 150°C for 30 seconds, when the thickener particles (b) are contained in the composition with 10% by mass.