A portable information processor in which heat generated internally is removed efficiently and noise incident to passage of an exhaust air flow generated from a cooling fan is damped in order to realize a silent information processor. The portable information processor comprises a body section including a CPU, a main memory, an input means, a display means, and the like, and an extension section including an extension power supply section, an external storage means, various interfaces, and the like, being mounted removably, the processor has an outlet for exhausting heat generated in the body to the outside thereof, a cooling fan generating an exhaust air flow being exhausted through the outlet, an inlet provided on the outer surface of an extension section, for introducing the exhaust flow generated by the cooling fan into the extension section corresponding to the outlet when the body section and the extension section are coupled, and a silencing means for damping a sound incident to passage of the exhaust air flow introduced from the inlet.