An image forming apparatus includes a developer unit, a toner container, a capacitance sensor, a toner amount detector, and a determining unit. The toner container is located to be removably attachable to the image forming apparatus. The toner container supplies a toner to the developer unit. The capacitance sensor is located on a supply passage for the toner from the toner container to the developer unit. The capacitance sensor detects an electric charge amount and a relative dielectric constant of the toner. The electric charge amount is an electric charge amount when the toner passes through the supply passage. The toner amount detector detects an amount of the toner passing through the supply passage based on the electric charge amount detected by the capacitance sensor. The determining unit determines whether the relative dielectric constant of the toner detected by the capacitance sensor is in a predetermined range.