An instrument used to measure the content of soil organic matter was developed based on spectroscopy. The instrument consists of an optical unit and an electronic unit. The optical unit is used to collect the reflecting light from soil surface and change the filtered light into the electrical signal. The electronic unit includes an amplifying circuit, an A/D circuit, and a display and storage circuit. Three wavebands of filters, 530, 765, and 850 nm, were selected in the performance test in order to determine the optimal one. The soil samples tested include two types, dried soil and air-dry soil. The result shows that there is highest correlation between the reflectance and the content of soil organic matter at the wavelength of 850 nm (r2 = 0.930) for dried soil. While the coefficient of determination r2 is abouu 0.711 for air-dry soil at the same wavelength. Although the actual precision is lower because of the soil moisture, it is practical in real-time measurement in the field.