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Hello HIRO-san,
It is OK to leave unused output and input pins disconnect (floating) on this device. If you wanted to reduce the chance that the unused channel toggles (i.e., based on noise present at the input), then you could use a pull-up resistor on one input line and a pull-down on the complementary input. Typical values would be 1 kOhm to 10 kOhm.
Max