The aim of this paper is' to formulate a rule which assigns the mora phoneme /Q/ to English loan words in Japanese. To achieve this aim, I will ' assume that morae should be classified into three types with respect to "weight" : normal mora consisting of CV is regarded as weight 1, super mora consisting of CVC or CV as weight 2, and ultra-super mora consisting of CVC as weight 3. It will be found from this assumption that /Q/ is inserted between two morae when the difference of the weight of the two morae is 1. In other words, when the weight difference is either the same or more than' 1, /Q/ is not inserted. I will formulate this finding as /Q/-Insertion Rule, and attempt to explain in terms of this rule why some words like hit and ' tap have /Q/ in their loan forms while others like carp and bike do not. In addition to /Q/-Insertion Rule, an extrametrical rule is also proposed which handles words like dress and tough exceptional to the insertion rule.