The purpose of this study is the study of a partially filled pipeline flowmeter for effective rainwater recycling. However, first flush have been contaminated with dirty dust, leaves, etc. on the roof of the building or the water surface of the house, which has hindered the rainwater utilization. Therefore, various methods for first flush diverter are used, but current technology has a problem that it does not work on drizzle or the amount of throwing away rainwater increases. In this study, we have studied a partially filled pipeline flowmeters which can utilize the rainwater effectively by using the contaminated first flush diverter by accurately measuring the amount of rainwater. The partially filled pipeline flowmeter has been developed for 75A, 100A and 150A for use only for rainwater use and has differentiated from conventional large diameter the partially filled pipeline flowmeter. The flowmeter developed in this study is a principle that calculates the flow rate by measuring the water level using buffer tank and Load Cell. To verify the accuracy of the flowmeter, we compared it with an electronic flowmeter that was calibrated with KOLAS. As a result, it was confirmed that the measurement accuracy of the partially filled pipeline flowmeters developed and developed with an error rate of 0.26% was high.