Abstract: A production method for a polymer comprises: a step (A) of polymerizing a raw material monomer to obtain a polymer-containing liquid containing a polymer; a step (B) of separating a collection target component-containing fluid containing a collection target component composed of at least one of a solvent and a residual monomer, from the polymer-containing liquid; a step (C) of supplying the collection target component-containing fluid to a three-product distillation column, and separating a fraction containing an impurity higher in boiling point than the collection target component and a fraction containing an impurity lower in boiling point than the collection target component, to obtain a fraction containing the collection target component of high purity; and a step (D) of performing polymerization using the fraction containing the collection target component of high purity.

Abstract: A spark plug includes: a rod-shaped center electrode extending in an axial direction; an insulator including a shaft hole that holds the center electrode inside of the shaft hole; a metal shell surrounding and holding a part of the insulator; and a ground electrode including a base end welded to the metal shell. The metal shell and the ground electrode are joined via a fusion portion formed by welding the ground electrode and the metal shell together. When the smallest thickness of the fusion portion in the axial direction is A, when, in a cross section that includes a center line of the ground electrode and is parallel to the axis, a length of a ground-electrode-side melted boundary formed between the fusion portion and the ground electrode is B, and when the ground electrode thickness is C, conditions of A?0.2 mm and B>C are satisfied.