IL-33 contributes to the pathogenesis of allergic diseases. Upon the binding of IL-33 to a membrane receptor ST2, IL-1 receptor accessory protein (IL-1RAcP) is recruited to form a heterodimeric receptor complex which transmits a signal. On the other hand, a soluble form of ST2 acts as an endogenous inhibitor of IL-33 signaling. Here, we aimed to develop the biologics inhibiting IL-33 signaling. First, we generated IL-33 reporter cell lines, in which IL-33 stimulation induced NFkB-driven expression of DsRed. We then generated IL-33trap-Fc which is composed of extracellular domains of IL-1RAcP and ST2 fused to the Fc portion of human IgG. Notably, IL-33trap-Fc more strongly suppressed IL-33-stimulated DsRed expression in the reporter cells than ST2-Fc which is composed of an extracellular domain of ST2 fused to human IgG Fc. Consistently, IL-33trap-Fc remarkably inhibited IL-33-stimulated IL-6 production of bone marrow-derived mast cells as compared with ST2-Fc. Moreover, intraperitoneal administration of IL-33trap-Fc efficiently inhibited IL-33-stimulated eosinophil accumulation in vivo. Taken together, these results indicated that IL-33trap-Fc was effective in inhibiting IL-33 signaling. We also attempt to generate the biologics that bind to ST2 but fail to recruit IL-1RAcP, thereby blocking IL-33 signaling.