This paper shows that there is a subclass of adjectives in Japanese called adjectival nouns, which consists of adjectives mostly of foreign origin, and this subset of the adjectives is required to undergo nominalization by the merger of a null nominalizing suffix. This process of nominalization takes place either prior to the PF interface or at PF, depending on the types of null nominalizing suffixes that merge with adjectival nouns. The proposed multilevel nominalization analysis has desirable consequences for adjectival nouns, and can also be extended to a subclass of verbs in Japanese, which likewise are typically of foreign origin.