Property of toner layer on a developing roller for monocomponent developing system has been investigated using a compact draw-off type instrument that measures toner charge and toner mass amount. The developing unit consists of a developing roller made of aluminum, a toner supply roller, and a blade, which controls the mass amount and the tribocharge of toner applied on the developing roller. The toner consists of polyester resin particles, which includes color agent, charge control agent, and external additives of silica. The charge measurement of toner on the developing roller is performed by directly absorbing toner particles with the absorption nozzle of the instrument. The toner mass amount on the roller surface is obtained simultaneously from the amount of toner absorbed into the absorption nozzle. The pushing pressure of the blade on the developing roller, the surface roughness of the roller, and external additives on toner surface affect the toner mass amount of the toner layer on the roller as well as the charge amount of the toner layer. It is found in this study that the charge to area ratio, which is a product of the toner charge and the toner mass amount on the developing roller, is very important to determine charging properties of toner particles.