The composite effects of p-type MgO and n-type MgFe2O4 on H2S detection were investigated. The mixture state of these oxides was fixed under coprecipitation conditions: mixture ratio of iron source to magnesium source, and pH control of precursor solution by dropping of an alkaline solution. The composite mixture of MgO, Fe2O3, and MgFe2O4 was obtained by adjusting the mixture ratio of Fe to Mg. The resistance in air monotonically increased with increasing MgO content in the matrix of MgFe2O4. In contrast, the resistance in air decreased with increasing Fe2O3 content. The sensing properties of these composites to 3 ppm H2S were evaluated as a sensor response (S), defined as the ratio of the resistance in air (Ra) to that in the gas mixture of H2S and air (Rg). The sensor response depended on the composite state of MgFe2O4 and MgO because the amount of H2S adsorbed on the composite was increased by the formation of the p–n junction. In contrast, the sensor response was not improved in the presence of excess MgO in the matrix of MgFe2O4, suggesting that the excess MgO serves as an insulator in the electron transfer between the semiconductor composite MgO-MgFe2O4 and an interdigitated Pt microelectrode.