In this paper, we propose and implement encounter data transmission with an ultrasonic sensor-based active wakeup mechanism for sparse wireless sensor networks (SWSNs), in which sensors are placed sparsely and each sensor is unable to communicate directly. We suppose that an active wakeup mechanism will be more suitable than a low-duty-cycle mechanism for SWSNs, since the collecting node moves around randomly in the sensing field. However, it was not clear whether the collecting node can communicate with the sensor in the short passing-through period. In this paper, we propose to use an ultrasonic sensor for waking up the communication function. We also succeed in developing a real-world sensor node that wakes up only when it detects the closing of the collecting node. We evaluate the detection ratio and the average communication duration of our system in a real-world agricultural application. As a result, we confirm that our system can provide stable communication between the collecting node and the sensor for at least 20 s at 10 kmph and for 10 s at 20 kmph.