A catalyst for bulk polymerization of a polymerizable unsaturated compound of the invention comprises a compound having at least one thiol group and a secondary hydroxyl group and represented by the following formula (I): ##STR00001## wherein R.sup.1 to R.sup.5 are each a hydrogen atom or an alkyl group, and R.sup.6 is a hydroxyl group, an alkyl group or the like. Also a process for bulk polymerization of the invention comprises polymerizing a polymerizable unsaturated compound in an inert gas atmosphere substantially in the absence of a solvent using, as a catalyst, a compound having at least one thiol group and a secondary hydroxyl group and represented by the above formula (I). According to the catalyst or the process of the invention, bulk polymerization can be carried out in one-component catalyst system in a high yield without suffering a runaway reaction.