Ralph never fully believes either way on the beast. He would like to think that there is no beast, but cannot fully convince himself that the beast is a fantasy.
At no point is Ralph's complete disbelief in a beast articulated.
In chapter six, Ralph has two reasons to think he won't see the beast during the action of the chapter. First, when he is going around the "castle" rock for the first time he notices certain elements of the landscape that suggest there is nothing living behind the rock. Second, the day is fading away and the boys are not quickly willing to go to the top of the mountain where the beast had been spotted by Sam and Eric. Night will come before there will be a chance to see the beast.
In chapter 6 ofThe Lord of the Flies, Ralph is initally afraid of the potential of beasts after the twins' startling report of seeing one. However, when Ralph goes to explore the ledge of the hill alone, he gains self-confidence.