## 1 Introduction  
There have been many cases of destruction originating from welds [1]. Therefore, further improvement in the reliability of welded part is required. At present, the quality evaluation of welds is performed after welding, in other words, post-processing, and when defects are detected, repair welding is performed again as necessary. For thin-plate welded joints, sampling inspection is generally performed because total inspection is not practical with conventional inspection methods, which include a destructive inspection. However, it is impossible to determine the presence or absence of defects in all welded parts by sample inspection. Therefore, according to the sampling inspection results, repairs are made or discarded on a lot-by-lot, resulting in huge costs. Thus, assurance by total inspection is required. For this purpose, a non-destructive inspection method applicable in situ, which can accommodate relatively high-speed welding for thin plates, is needed. In particular, galvanized steel sheets are in high demand in many fields such as for automobile body panels because of their superior corrosion resistance, lightweight, and economic efficiency. However, galvanized steel sheets are prone to pore defects called blowholes in the overlapped area in lap joint welding [2]. This is due to the evaporation of the galvanized layer during welding, and the gases generated penetrate the molten pool and remain there. Although it is possible to reduce the occurrence of blowholes by creating gaps in the overlap area, it is difficult to control the gaps. There is a great need for the quality assurance of galvanized steel sheets. In-process monitoring for weld quality including the presence or absence of weld defects in parallel with the welding process is very effective in meeting these requirements for welding galvanized steel sheets with lap joint welding. In-process monitoring requires establishing a measurement method that can be applied in high-temperature and high-noise environments during welding. However, conventional non-destructive inspection methods such as ultrasonic testing (UT) require a transducer to contact the measurement target to acquire internal information, making it difficult to apply and automate such methods in high-temperature environments. Thus, a non-contact method is desirable. Several methods can be used for this purpose such as the X-ray method [3], an ultrasonic method using buffer rods [4], electromagnetic ultrasonic method [5], airborne ultrasonic method [6], and laser ultrasonic method [7]. Among them, the laser ultrasonic method, in which ultrasonic waves are excited by irradiating a pulsed laser onto the target surface, and the ultrasonic waves propagating inside or on the surface and reflected at various interfaces are received by a laser interferometer, is suitable for in-process monitoring due to the following features. (1) Using a laser as a probe allows non-contact measurement with the object. (2) The point-to-point measurement by lasers makes it applicable to narrow areas. (3) Optical mirrors and mechanical stages can scan the lasers, and information on ultrasonic propagation at multiple points can easily be obtained. With these features, it has been used for defect measurement [8; 9] including the detection of internal defects [10; 11; 12; 13]. In addition, there have been many research reports on plate thickness measurement [14] and material evaluation [15; 16]. On the other hand, there are no reports of the in-process detection of internal defects in thin-plate welds. Most of the experiments on thin-plate components are limited to specimens with simplified geometry, and there are only a few experiments on parts with complex geometry such as weld beads and weld reinforcement. However, if an in-process monitoring technique using the laser ultrasonic method can be established for thin-plate welded components, reliable quality assurance by total inspection can be realized.