International simple glass (ISG) is a six-component alumino-borosilicate glass developed as a reference benchmark glass for six nations collaborating study on high-level nuclear waste glass dissolution/corrosion mechanism. In this study, aqueous dissolution tests were performed for the ISG using microchannel flow-through (MCFT) method to evaluate the initial dissolution rate of glass matrix, r0, precisely and systematically as a function of solution pH and temperature. The test results indicated that the r0 shows a “V-shaped” pH dependence with a bottom at around pH4 at each temperature. Compared with Japanese reference glass of P0798, for which the r0 showed a “U-shaped” pH dependence with a bottom at around pH6 in our previous study, the ISG shows the higher dissolution rate at basic pH, and lower dissolution rate at neutral to acidic pH. The results also indicated that the r0 increases with temperature according to an Arrhenius law, and the apparent activation energy evaluated from Arrhenius relation is 62–77 [kJ/mol] at any pH from 3 to 10, which suggests the initial dissolution of ISG proceeds controlled by a surface-reaction mechanism in this pH range.