In this paper, the method for recognition of lawn information in mowing robots (Robomower) is proposed. The mowing robots will be expected to work in open areas such as parks. To work in an open area, the robots must keep the safety of the public. It is necessary to collect surrounding information adequately. The Robomower is a small and low-power robot, which uses GPS information to get coordinates of its current position. It has many sensors: GPS, terrestrial magnetism direction sensor, ultrasonic wave sensors, tactile bumper switches etc. However, the robot did not have the means to measure the height of the grass. Reflection type sensors and image analysis system are not effective for this task. We need to recognize the lawn to complete mowing. We succeeded in measuring grass height with a simple, cheap sensor. The method is to estimate the height of the turf by using a transmission type photointerrupter. The effectiveness of the sensor was confirmed by experimentation.