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Here is a piece of text from 「ノルウェイの森」by Haruki Murakami:
この寮の唯一の問題点はその根本的なうさん臭さにあった。寮はあるきわめて右翼的な人物を中心とする正体不明の財団法人によって運営されており、その運営方針は――もちろん僕の目から見ればということだが――かなり奇妙に歪んだものだった。
How are these things before もちろん and かなり called and what function they serve?
Is it a mark equal to 、 or something informal like ~~~ and expressing a short speech pause in a middle of the sentence? In any case, what is its name?
It is called a "dash", ダッシュ in Japanese, according to this wikipage.
Several usages of the symbol are listed in the page. The usage here is to insert an extra explanation to the sentence (or word before the first dash). You can see that if you take off everything between the two dashes, the sentence is perfectly correct and meaningful. The portion between the two dashes is only to add an extra explanation that the statement is purely the writer's own opinion. You can treat it as a pair as bracket "()" here.
In English typography, there is a practice to substitute two short dashes in order to simulate a long one -- like this -- with or without surrounding spaces, when the stock character is not available in the given typesetting system.
What you have there looks like an imitation of the practice, using kanji style dashes. The writer probably thought, "I want to use this neat thing seen in Western writing, in my native writing".
It looks like a Western influence, akin to the main stream use of roman numerals, or punctuation like exclamations and question marks.
