A developing device includes a housing for housing therein a developer containing toner and magnetic carrier, and a first sensor including an oscillation circuit and a coil board which is part of the oscillation circuit and on which a coil pattern is formed. A fitting portion formed by recessing an outer surface of the housing is provided in the housing so as to allow the coil board to be fitted at a position closer to the developer than the outer surface of the housing. The coil board is fitted into the fitting portion.