本文指出，中古漢語有一個新興的平比句乙式（「X＋如＋Y＋A」式），當時主要是流行於南方，且有取代上古漢語所流行的平比句甲式（「X＋A＋如（若）＋Y」式）之勢。在中古漢語時期，雖也有形式如同上古甲式的句式，可是因為經過重新分析而導致句構與上古漢語有異。本文還指出，乙式之所以取代甲式，中古漢語名語化記號的消失以及反義並列複合詞的興盛都是重要的因素。

In this paper, we show that the sentence pattern “X + RU (RUO) + Y + Adj” ‘X + as + Y + Adj’ with the meaning of a certain equal degree between X and Y, which newly developed in Middle Chinese and distributed mainly in the southern part of the country, gradually replaced the earlier sentence pattern “X + Adj + RU (RUO) +Y” ‘X + Adj + as + Y’, which prevailed in Old Chinese and denotes a similar meaning to the former sentence pattern. In this paper, we call the old sentence pattern “type B” and the new sentence pattern “type A”. Although there are examples in Middle Chinese, which are in the form of type A in Old Chinese, they are in fact different from type A in structure because of the way of reanalysis. As for the mechanisms by which type B replaced type A, we suggest there are at least two factors that might be responsible for it: the disappearance of the markers of nominalization and the prosperity of compounds of antonyms in Middle Chinese.