Irfan View公式ページのFAQに答えが載っています。
IrfanView Frequently Asked Questions
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Q: Problem: I can't set IrfanView as my default viewer on Windows 11??
A: There seems to be a system problem in some Win11 installations.
Try this: start System Settings, "Apps and Features", find "Microsoft Photos". Click on the menu on the far right, chose "Advanced options" go to the Reset section and perform both the Repair and the Reset (don't start "Microsoft Photos" immediately after this). Now, associate JPG etc. with IrfanView.
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