I'm not so stupid as to believe that.
(そんなことを信じるほど私はバカではありません)
「独立不定詞」
全て熟語として覚えておきましょう:
to be honest [frank]
(正直[率直]に言うと)
to tell (you) the truth
(本当のことを言うと)
not to mention ...
(...は言うまでもなく)
to say nothing of ...
(...は言うまでもなく)
to say the least (of it)
(控えめに言っても)
not to say ...
(...とは言わないまでも)
needless to say
(言うまでもなく)
so to speak
(言うなれば)
to begin [start] with
(まず始めに)
to make matters worse
(さらに悪いことには)
(正直[率直]に言うと)
to tell (you) the truth
(本当のことを言うと)
not to mention ...
(...は言うまでもなく)
to say nothing of ...
(...は言うまでもなく)
to say the least (of it)
(控えめに言っても)
not to say ...
(...とは言わないまでも)
needless to say
(言うまでもなく)
so to speak
(言うなれば)
to begin [start] with
(まず始めに)
to make matters worse
(さらに悪いことには)