An image forming apparatus which is capable of bringing processing devices, such as sheet feed devices and post-processing devices connected respectively to the image forming apparatus, into operable states in timing optimal for carrying out an image forming job, thereby improving productivity and operation rate of the image forming apparatus as well as saving energy. The aging period is based on a proportional utilization level of the cache memory by the write-back data, the higher the memory utilization level, the shorter the period for aging the write-back data. The plurality of mobile devices are interconnected with the server via the carrier network and the network and are capable of communicating with each other via the server. The input and the output power signals of the resonance module are derived and information bearing parts of the derived power signals are divided with each other to produce a ratio. The shelters or housing units may be deployed in emergency housing situations and may be grouped into systems of shelters.