Category-based inferences were divided into two types: the downward inference which the category terms in the conclusion are lower than those in the premise and the upward inference which the former terms are higher than the latter ones. Three tasks were provided for each inference. Kindergartners, second graders, and college students were required to answer to the conclusion statements with "Yes" or "No" and to justify the answers. The mean number of the positive answers ("Yes") increased with ages for the downward inference while for the upward inference it was the largest for the 2nd graders. For the students the mean number was significantly greater for the downward than for the upward inferences. The findings were discussed with acquisition and use of three types of knowledge; for the downward inference the inclusive knowledge that the conclusion category terms are included in the premise ones, and for the upward inference the associative knowledge that the premise category terms are associated to the conclusion ones and the inclusive knowledge that the conclusion category terms include some category terms other than the premise ones.