A duty ratio Du (%), denoted by (T2/(T1+T2))×100, is in the range of 60≦Du≦80; a magnetic carrier has a resistivity ρ which decreases in accordance with an increasing electric field strength, and a relative dielectric constant ∈ which increases in accordance with an increasing electric field strength; a product of a time constant ∈0∈ρ(s) of electric charge decay in an electric field strength E2D decided by a second peak voltage V2 and a dark potential VD, and an electric field strength E2D satisfies a relation of 20≦∈0∈ρ E2D (s·V/cm); and a time constant ∈0∈ρ(s) and a relative dielectric constant ∈ in an electric field strength E1L, which is decided by a first peak voltage V1 and a bright potential VL, satisfy the following relations: ∈0∈ρ(s)≦6.0×10−4, and 30≦∈.