Anaerobic digestion is widely used for waste treatment and for biofuel production in the form of biogas from dedicated energy crops. Proper operation and control of anaerobic digesters require knowledge of biogas flow rate and methane composition of biogas. Methane content of biogas from an anaerobic digester was measured on-line by modifying an off-line measurement device, previously constructed by the authors, that uses a hydrocarbon sensor (MQ-4) and a pressure/temperature/humidity sensor (BME-280) integrated with an Arduino Uno. This modified on-line sensor was programmed to automatically measure methane composition by self-regulated introducing biogas sample and evacuating the device. It also equipped with a function of biogas flow chamber which could calculate the cumulative biogas production over a time interval. Methane composition (v/v) could be measured every three hours. Measurements made by this device agreed within an average absolute difference of 0.81 ± 0.58 % with measurements by a gas chromatograph equipped with a thermal conductivity detector. The volumetric flow rate was directly expressed on a moisture-free basis at standard temperature and pressure. The total cost of this device was under US $140. For an additional US $7, the device can be upgraded with an LED display that toggles between composition and flow rate.