A press-forming device includes a punch and a die, as a mold to be measured, and a strain measurement means arranged within the mold and measuring a strain amount generated by press-forming. Thus, the priming effect in the following discharge is generated. A method of rubbery polymer synthesis, in one embodiment, involves mixing anionically polymerizable conjugated diene monomers with optional vinyl aromatic and an anionic-polymerization initiator, i.