We develop a new measurement system which can measure position and orientation of the end-effector of a six-axis welding robot. The developed measurement system consists of five digital probes. The measurement values from the digital probes are transformed into position and orientation of the end-effector with consideration of measurement system kinematics. Calibration procedure is applied to the probe system and accuracy of the system is measured. After the calibration, the positional accuracy is observed as 0.025mm, and the orientational accuracy is $0.075^{circ}$, respectively. By using the developed measurement system, we present an experimental result for controller gain tuning about a welding robot. We used Taguchi method to find optimal gain set and succeeded to suppress the fluctuation of the end-effector. The fluctuation with high frequency can be reduced by 54% after gain tuning.