The second-order wave equation $\frac{{\partial }^{2}u}{\partial x_i\partial x^i}=0\mathrm{}$ in five-dimensional flat space can reproduce approximately the field hitherto considered in four-dimensional space, if a de Sitter space with the radius $R$ is imbedded in the five-dimensional flat space and the derivative of the wave function $u$ in the direction normal to this de Sitter space is assumed to be equal to imu, where $m$ is a constant proper to the field. Conversely, the ordinary field equations in four-dimensional space can be regarded as valid only for a portion of de Sitter space with the linear dimension, which is small compared with the radius $R$ of the universe. Extensions of these arguments to electromagnetic field and spinor field are also considered.

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