A method of producing an ethylene-propylene-diene elastomer; wherein the method comprises a step of polymerizing ethylene, propylene, and a diene monomer in the presence of a catalyst composition comprising a transition metal compound as represented by Chemical Formula 1; wherein the ethylene-propylene-diene elastomer has a density of 0.850 ∼ 0.895 g/cm 3; and the ethylene-propylene-diene elastomer has an ethylene content of at least 20% by weight but less than 90% by weight, a propylene content of 10 to 80% by weight, and a diene monomer content of greater than 0 but no more than 20% by weight: in Chemical Formula 1, R1 and R2 are the same with or different from each other, and are independently hydrogen; a C1 to C20 alkyl radical; a C2 to C20 alkenyl radical; a C6 to C20 aryl radical; a silyl radical; a C7 to C20 alkylaryl radical; a C7 to C20 arylalkyl radical; or a metalloid radical of Group IV metal substituted with a hydrocarbyl group, respectively; and R1 and R2 or two R2s can be linked to each other by an alkylidine radical including a C1 to C20 alkyl or a C6 to C20 aryl radical to form a ring; R3s are the same with or different from each other, and are independently hydrogen; a halogen radical; a C1 to C20 alkyl radical; a C2 to C20 alkenyl radical; a C6 to C20 aryl radical; a C7 to C20 alkylaryl radical; a C7 to C20 arylalkyl radical; a C1 to C20 alkoxy radical; a C6 to C20 aryloxy radical; or an amido radical, respectively; and two or more of the R3s can be linked to each other to form an aliphatic ring or an aromatic ring; CY1 is an aliphatic or aromatic ring, unsubstituted or substituted with a halogen radical; a C1 to C20 alkyl radical; a C2 to C20 alkenyl radical; a C6 to C20 aryl radical; a C7 to C20 alkylaryl radical; a C7 to C20 arylalkyl radical; a C1 to C20 alkoxy radical; a C6 to C20 aryloxy radical; or an amido radical, and when it has a plurality of the substituents, two or more of them can be linked to each other to form an aliphatic ring or an aromatic ring; M is a transition metal of Group IV; and Q1 and Q2 are the same with or different from each other, and are independently a halogen radical; a C1 to C20 alkyl radical; a C2 to C20 alkenyl radical; a C6 to C20 aryl radical; a C7 to C20 alkylaryl radical; a C7 to C20 arylalkyl radical; a C1 to C20 alkyl amido radical; a C6 to C20 aryl amido radical; or a C1 to C20 alkylidene radical, respectively. R1 and R2 are the same with or different from each other, and are independently hydrogen; a C1 to C20 alkyl radical; a C2 to C20 alkenyl radical; a C6 to C20 aryl radical; a silyl radical; a C7 to C20 alkylaryl radical; a C7 to C20 arylalkyl radical; or a metalloid radical of Group IV metal substituted with a hydrocarbyl group, respectively; and R1 and R2 or two R2s can be linked to each other by an alkylidine radical including a C1 to C20 alkyl or a C6 to C20 aryl radical to form a ring; R3s are the same with or different from each other, and are independently hydrogen; a halogen radical; a C1 to C20 alkyl radical; a C2 to C20 alkenyl radical; a C6 to C20 aryl radical; a C7 to C20 alkylaryl radical; a C7 to C20 arylalkyl radical; a C1 to C20 alkoxy radical; a C6 to C20 aryloxy radical; or an amido radical, respectively; and two or more of the R3s can be linked to each other to form an aliphatic ring or an aromatic ring; CY1 is an aliphatic or aromatic ring, unsubstituted or substituted with a halogen radical; a C1 to C20 alkyl radical; a C2 to C20 alkenyl radical; a C6 to C20 aryl radical; a C7 to C20 alkylaryl radical; a C7 to C20 arylalkyl radical; a C1 to C20 alkoxy radical; a C6 to C20 aryloxy radical; or an amido radical, and when it has a plurality of the substituents, two or more of them can be linked to each other to form an aliphatic ring or an aromatic ring; M is a transition metal of Group IV; and Q1 and Q2 are the same with or different from each other, and are independently a halogen radical; a C1 to C20 alkyl radical; a C2 to C20 alkenyl radical; a C6 to C20 aryl radical; a C7 to C20 alkylaryl radical; a C7 to C20 arylalkyl radical; a C1 to C20 alkyl amido radical; a C6 to C20 aryl amido radical; or a C1 to C20 alkylidene radical, respectively.