We consider the initial-boundary value problem of semilinear wave equation with nonlinearity |u| p in exterior domain in R N (N ≥ 3). Especially, the lifespan of blowup solutions with small initial data are studied. The result gives upper bounds of lifespan which is essentially the same as the Cauchy problem in R N . At least in the case N = 4, their estimates are sharp in view of the work by Zha-Zhou [21]. The idea of the proof is to use special solutions to linear wave equation with Dirichlet boundary condition which are constructed via an argument based on Wakasa-Yordanov [15]. Mathematics Subject Classification (2010): Primary: 35L05, 35L20, 35B44.