A developing device includes a container that contains toner particles
that are for developing a latent image borne by an image bearing body,
and a toner-particle bearing roller that has a helical groove portion on
a surface thereof that is for bearing the toner particles, the helical
groove portion having an inclination with respect to an axial direction
and a circumferential direction of the toner-particle bearing roller and
being formed having a uniform pitch in the axial direction.