In protected crop production facilities such as greenhouse and plant factory, farmers should be present and/or visit frequently to the production site for maintaining optimum environmental conditions and better production, which is time and labour consuming. Monitoring of environmental condition is highly important for optimum control of the conditions, and the condition is not uniform within the facility. Objectives of the paper were to investigate spatial and vertical variability in ambient environmental variables and to determine optimum sensor locations. Experiments were conducted in a strawberry-growing greenhouse (greenhouse 1) and a cherry tomato-growing greenhouse (greenhouse 2). Selected ambient environmental variables for experiment in greenhouse 1 were air temperature and humidity, and in greenhouse 2 were air temperature, humidity, luminous intensity, and CO₂ concentrations. Variability in air temperature and humidity at noon was the highest, and reduced over time. Locations of measured maximum and minimum values were changed by window-opening and heating operation. Scenarios for the location of luminous intensity and CO₂ concentration sensors in greenhouse were suggested considering the variability, and also control methods.