To detect, with high precision, a small amount of splashes during joining by resistance welding the components to be contained into a sealed type secondary battery to enhance reliability on the product.

A moving average resistance value Rav(j) subject to only comparatively linear variation is calculated from a moving average voltage Vav(j) and a moving average amperage Iav(j) at a moving average resistance value calculation section 243, instead of voltage and amperage which greatly vary due to inverter control. At a splash judging section 244, whether or not splashes occur is judged on the basis of whether the moving average resistance value Rav(j) exceeds a threshold value Rth(j). This can bring the time variation of the threshold value near to the time variation of the moving average resistance value for easy setting, and it is feasible to detect even a small amount of splashes occurring in the last period of welding, with the moving average resistance value increasing gradually.