NOVELTY - A curing agent comprises particles, and are solid at 25 degrees C. Undersize cumulative fraction of 50% particle diameter (D50) of the particles is 0.3-12 mu m. The particle size distribution of particles is ratio of undersize cumulative fraction of 50% particle diameter (D50) of the particles with respect to undersize cumulative fraction of 99% particle diameter (D99) of the particles, and is 6 or less. The specific surface area value (y) and particle size D50(x) of particles satisfy a preset relation (I). The moisture content of the curing agent is 1.5% or less.    USE - Curing agent is used in curing agent/one component epoxy resin composition for forming molded article such as electroconductive material, anisotropically conductive material, anisotropically conductive film, insulating material, sealing material, coating material, coating composition, prepreg, thermally conductive material, separator material for fuel cells and overcoat material for flexible wiring boards (all claimed).    ADVANTAGE - The curing agent has excellent hardenability, storage stability and clearance gap permeability. Aggregation of particles of the curing agent is reduced.    DETAILED DESCRIPTION - A curing agent comprises particles, and are solid at 25 degrees C. Undersize cumulative fraction of 50% particle diameter (D50) of the particles is 0.3-12 mu m. The particle size distribution of particles is ratio of undersize cumulative fraction of 50% particle diameter (D50) of the particles with respect to undersize cumulative fraction of 99% particle diameter (D99) of the particles, and is 6 or less. The specific surface area value (y) and particle size D50(x) of particles satisfy the relation: 4x-1 less than or equal to y less than or equal to 8.3x-1 (I). The moisture content of the curing agent is 1.5% or less. INDEPENDENT CLAIMS are included for the following:    (1) curing agent (c2) for microcapsule-type epoxy resin, which comprises core containing the curing agent, and shell containing bonding groups (x-z), which covers core. The bonding group (x) absorbs infrared rays at 1630-1680 cm-1. The bonding group (y) absorbs infrared rays at 1680-1725 cm-1. The bonding group (z) absorbs infrared rays at 1730-1755 cm-1;    (2) curing agent composition, which comprises curing agent (c2) and epoxy resin (e3), in mass ratio of 100:0.1-100:1000; and    (3) one-component epoxy resin composition, which comprises the curing agent composition and epoxy resin (e4), in mass ratio of 100:0.001-100:1000.