The author studies the generation problem of the \(C_0\)-semigroup of the free streaming operator with noncontractive boundary operators. The paper is mainly composed of six sections. In section 2, the author presents some boundary conditions such as the local and nonlocal boundary condition as well as nonlocal Maxwell-type boundary conditions which are commonly adopted in the kinetic theory of gases and in population dynamics. In section 3, the author introduces the functional setting and proves the classical generation theorem for contractive boundary conditions. In section 4, the author presents the so-called phase space approach. Here he splits the phase spaces into two types: one is a regular phase space in which the free streaming operator always generates a semigroup without any assumption on the boundary operator, another is a nonregular phase space. In section 5, the author studies the influence of the boundary operator in nonregular phase space. Under certain smallness assumptions on the boundary operators, the author proves that the corresponding streaming operator also generates a \(C_0\)-semigroup by using a trick of operator similarity. In section 6, the author applies the result obtained in section 5 to the Maxwell-type boundary conditions and obtains the result of generation of a \(C_0\)-semigroup for the boundary condition aforementioned in section 2. The author ends the present paper by a concluding remark and a conjecture.