The invention relates to particles with an average particle size of 300 to 1000 μm, comprising a modified polyvinyl alcohol having vinyl alcohol units and structural units represented by formula (1), wherein a content of the vinyl alcohol units is 60 to 90 mol % based on the total structural units: wherein R1 represents a hydrogen atom or a methyl group. Thus, the present invention provides particles containing a modified polyvinyl alcohol having high water solubility and high affinity for a photosensitive monomer, which are highly photosensitive.