An image forming device includes: an endless intermediate transfer belt; and a secondary transferrer, wherein the secondary transferrer includes a secondary transfer roller and a counter roller, a value calculated from w/Vsys is defined as a nip time N [sec], when the secondary transferrer is deemed as an equivalent circuit including a resistance R1 [Ω·m2] of the secondary transfer roller, a resistance R2 [Ω·m2] of the intermediate transfer belt, a resistance R3 [Ω·m2] of the counter roller, and capacitance Cmed [F/m2] of a recording medium, in a case where a combined resistance in the equivalent circuit is defined as Rtotal [Ω·m2], and combined capacitance in the equivalent circuit is defined as Ctotal [F/m2], a value calculated from Rtotal×Ctotal is defined as a time constant τ [sec], and the image forming device further includes: a sheet detector; and a hardware processor.