Method of inspecting a presence or absence of an assembly defect of a gas sensor into which a sensor element is incorporated inside, wherein said gas sensor comprises: a first gas sensor constituting member having an insertion opening into which said sensor element is inserted; and a second gas sensor constituting member including said sensor element which partially protrudes from said second gas sensor constituting member, in said gas sensor, a part of said sensor element which protrudes from said second gas sensor constituting member is inserted into said insertion opening, so that said first gas sensor constituting member and said second gas sensor constituting member are integrated with each other, said sensor element includes a heater made up of a resistance heating member therein and also includes an electrode terminal for a heater in a surface thereof, said first gas sensor constituting member includes a contact point member which contacts said electrode terminal in a state where said sensor element is inserted into said insertion opening, and said method comprises steps of: determining an identification information enabling a unique identification of said sensor element; measuring a resistance value of said heater of said sensor element before incorporated into said gas sensor to obtain a first resistance value and associating said first resistance value with said identification information for said sensor element; measuring said resistance value of said heater via at least said contact point member, in a state where said first gas sensor constituting member and said second gas sensor constituting member are integrated with each other, to obtain a second resistance value and associating one of said second resistance value with said identification information for said sensor element; and comparing a difference value between said first and second resistance values associated with an identical identification information with a predetermined threshold value, and when said difference value exceeds said predetermined threshold value, determining that an assembly defect occurs in said gas sensor into which said sensor element, to which said identical identification information is provided, is incorporated. a first gas sensor constituting member having an insertion opening into which said sensor element is inserted; and a second gas sensor constituting member including said sensor element which partially protrudes from said second gas sensor constituting member, determining an identification information enabling a unique identification of said sensor element; measuring a resistance value of said heater of said sensor element before incorporated into said gas sensor to obtain a first resistance value and associating said first resistance value with said identification information for said sensor element; measuring said resistance value of said heater via at least said contact point member, in a state where said first gas sensor constituting member and said second gas sensor constituting member are integrated with each other, to obtain a second resistance value and associating one of said second resistance value with said identification information for said sensor element; and comparing a difference value between said first and second resistance values associated with an identical identification information with a predetermined threshold value, and when said difference value exceeds said predetermined threshold value, determining that an assembly defect occurs in said gas sensor into which said sensor element, to which said identical identification information is provided, is incorporated.