To provide a medium suitable for high speed/high resolution, rich in hydrophilicity and resistant to a high concentration aqueous alkaline solution.A medium comprising crosslinked polymer particles containing from 20 to 95 mol % of repeating units derived from a (meth)acryloyl monomer represented by the formula (1): wherein R2 is a hydrogen atom or a C1-4 alkyl group, R1 is —NR3-R4-R5 or —O—R4-R5, R3 is a hydrogen atom or a C1-4 alkyl group, R4 is a C6-15 alkylene group containing an alicyclic ring or a C4-8 linear alkylene group, and R5 is a halogen atom, an alcoholic OH group, an amino group, a glycidyl group or an epoxy group.