The Japanese comparative expression kurabe mono-ni nara-nai ‘cannot be compared’ has some characteristics that ordinary comparatives lack. First, its meaning is ambiguous in terms of whether the subject x is much higher than the object y or whether x is much lower than y in terms of scale. Second, it always appears with negation. I will argue that these two kinds of interpretation can be derived from the interaction with negation and the notion of category, and that the polarity sensitivity of the expression is due to its interaction with the maxim of quantity (Grice 1975). I will also compare the Japanese data to some related expressions in Chinese, English, and Japanese and discuss their similarities and differences. This paper shows that there is a new type of comparative, context-dependent comparison in natural language whose relative relationship is not linguistically encoded explicitly.