The diffractive optical element includes a first diffraction grating and a second diffraction grating, which are formed of materials different from each other, the first diffraction grating and the second diffraction grating are stacked so as not to provide any air layer therebetween. The pressure relief device as claimed in claim 16, wherein the body further includes a retainer configured to prevent the valve from separating from the body when the cap becomes disengaged from the body. A packet transfer unit that can, upon receiving a packet that is destined for the home address of the mobile node, transfer a capsulated packet that is generated by attaching to the received packet a header that indicates a destination address by a temporary address that is correlated with a home address according to address information that is stored in the home migration host list hold unit. Also, in some embodiments, the generated sub-network performs a set of two or more functions.