shape such as square prism next operation of a fuel vapor treating system including the adsorbent canister 10 will be described fig1 the fuel vapor treating system is composed of the adsorbent canister 10 the vapor pipe 26 the fuel tank 27 the purge pipe 30 the air intake pipe 32 and purge valve 34 etc when the internal combustion engine 31 is stopped the fuel vapor e g vaporized in the fuel tank 27 flows into the main chamber 17 via the vapor pipe 26 then most of the fuel vapor adsorbs onto the adsorbent 42 filled in the main chamber 17 remaining fuel vapor that has not adsorbed onto the adsorbent 42 in the main chamber 17 flows into the auxiliary chamber 18 via the communication chamber 20 and then adsorbs onto the adsorbent 42 filled in the auxiliary chamber 18 in this state increase in temperature of the adsorbent 42 caused by exothermal reaction where the fuel vapor adsorbs onto the adsorbent 42 can be suppressed by endotherm caused by phase change from solid phase to liquid phase of the phase change material in the heat storage capsules of the shaped heat storage material 44 thus it is able to improve adsorption performance of the adsorbent 42 for the fuel vapor on the other hand in a state that the internal combustion engine 31 is running when the purge valve 34 is opened negative pressure in the internal combustion engine 31 can act on the adsorbent canister 10 due to this action ambient air flows into the auxiliary chamber 18 via the air communicating port 24 the air introduced into the auxiliary chamber 18 desorbs the.