Use the Generalise transformer. Only problem is it will smooth the entire polygon, not just the "ends".

Thanks for your response. I tried the result was not satisfied.

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@Takashi Iijima​ 

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Please can advise for to achieve.​

It might require a bit of work, but if you can extract the 3 end points on each end of this shape, which is easily done, you can use an arc estimator with these 3 points, or a 2DArcReplacer if you have more informations and want it a bit more precise.

And after you get the 2 arcs needed, you just assemble them with your output, as "Arcs and ellipses will be stroked prior to the calculation of the hull.", so not with the input.

If you know how to fill the blank/empty space, use the transformer, I don't know if there is one

If you don't, you can fill it with a custom transformer using python (might require multiple calls)

https://matplotlib.org/3.2.1/api/_as_gen/matplotlib.pyplot.fill_between.html

And finally use the dissolver to erase any interior leftovers

Sounds tortured, and it is, but it should work. There might be and easier way.

Hope this helps