A method of manufacturing a plastic lens, comprising: mixing components and, the component being an isocyanate terminal prepolymer in a form of a reaction product of an aliphatic diisocyanate having an intramolecular cyclic structure and a diol having an average molecular weight of 300-2,500, the component being at least one aromatic diamine denoted by a formula having a structure where in the formula, R 1, R 2 and R 3 are each independently one of a methyl, ethyl and thiomethyl group; casting, immediately after mixing, a mixture of said components and into a casting mold; and polymerizing the mixture to obtain a molded article, wherein the mixing of the components and is carried out after heating said component to reduce viscosity of said component, said casting mold has a mold for forming one side of the lens and a mold for forming the other side thereof that are positioned opposite at a prescribed interval, a circular gasket is positioned around said molds, and a cavity is formed by said molds and gasket, said gasket has a casting hole for casting the mixture of said components and into said cavity and a discharge hole for discharging gas and said mixture in said cavity to an exterior of the casting mold, that are positioned opposite in a diameter direction, said casting into the casting mold is carried out in a state that said casting mold is tilted from or perpendicular to a horizontal plane as well as said discharge hole is positioned at a top, and said polymerizing comprises leaving the casting mold undisturbed for 5 to 20 minutes in an atmosphere with room temperature, to polymerize said mixture in the casting mold by a self-heating of said mixture, and then leaving the casting mold in a high-temperature atmosphere at a temperature of 110° C to 150° C for 15 to 24 hours to further proceed polymerization of said mixture. mixing components and, the component being an isocyanate terminal prepolymer in a form of a reaction product of an aliphatic diisocyanate having an intramolecular cyclic structure and a diol having an average molecular weight of 300-2,500, the component being at least one aromatic diamine denoted by a formula having a structure where in the formula, R 1, R 2 and R 3 are each independently one of a methyl, ethyl and thiomethyl group; casting, immediately after mixing, a mixture of said components and into a casting mold; and polymerizing the mixture to obtain a molded article, wherein the mixing of the components and is carried out after heating said component to reduce viscosity of said component, casting, immediately after mixing, a mixture of said components and into a casting mold; and polymerizing the mixture to obtain a molded article, wherein the mixing of the components and is carried out after heating said component to reduce viscosity of said component, said casting mold has a mold for forming one side of the lens and a mold for forming the other side thereof that are positioned opposite at a prescribed interval, a circular gasket is positioned around said molds, and a cavity is formed by said molds and gasket, said gasket has a casting hole for casting the mixture of said components and into said cavity and a discharge hole for discharging gas and said mixture in said cavity to an exterior of the casting mold, that are positioned opposite in a diameter direction, said casting into the casting mold is carried out in a state that said casting mold is tilted from or perpendicular to a horizontal plane as well as said discharge hole is positioned at a top, and said polymerizing comprises leaving the casting mold undisturbed for 5 to 20 minutes in an atmosphere with room temperature, to polymerize said mixture in the casting mold by a self-heating of said mixture, and then leaving the casting mold in a high-temperature atmosphere at a temperature of 110° C to 150° C for 15 to 24 hours to further proceed polymerization of said mixture.