A detection device configured to detect a charge amount of toner on a developer carrier includes an assembly with a quartz oscillator and having a first surface and a second surface that are provided in the quartz oscillator. The first surface has a recess and a projection formed in a direction in which the quartz oscillator vibrates. In addition, a capacitor is connected to the assembly, a first measurement unit measures a potential difference between two ends of the capacitor, and a second measurement unit measures a frequency of the quartz oscillator. In a case where an alternating-current power is applied to the assembly, the direction in which the quartz oscillator vibrates is parallel to the first surface.