an airtight packaging apparatus for covering with a cap a multichip module having lsi chips mounted on a ceramic substrate and for sealing the cap and substrate by means of solder includes a receptacle for accommodating the multichip module , valves for supplying gas to the receptacle and exhausting the receptacle , a retainer for holding the cap and adjusting the positional relation with respect to the multichip module , and a heater for heating solder of the junction between the multichip module and the cap . after solder has been heated and melted , the multichip module and the cap are sealed to each other .