With respect to the reaction to fire performance of sandwich panel products, in ISO/TC92/SC1/WG7 “Large and intermediate scale fire test methods,” ISO 13784-1 “Reaction to fire test for sandwich panel building systems—Part 1: Small room test” is specified as a method of test for determining the reaction to fire behavior of sandwich panel building systems. On the other hand, according to EN 13501-1 “Fire classification of construction products and building elements,” EN 13823 (SBI test) and EN ISO 11925-2 (ignitability test) are used especially for evaluating the classes B, C, and D in the Euroclass system. The authors have developed a new intermediate-scale box test for sandwich panel products, pertaining to the correlation with ISO 13784-1 especially on the flashover occurrence, which was locally standardized as JIS A 1320:2017, and is now internationally being discussed as ISO/WD TS 23657 at ISO/TC92/SC1/WG7. In this paper, regarding five sandwich panel specimens, various fire tests were conducted according to ISO 13784-1, EN 13501-1, and ISO/WD TS 23657, and those results were compared with each other, indicating that ISO/WD TS 23657 has a good correlation with ISO 13784-1 in terms of flashover, and that evaluation results based on ISO/WD TS 23657 could be similar to those based on EN 13501-1. Furthermore, a new tentative classification system was proposed based on those limited experimental results conducted in this research.