a housing structure of a keyboard instrument has : a musical instrument main body having a keyboard portion in which a plurality of keys are aligned in order ; a cover having a hinge portion and a foldable portion along an alignment direction of the keys so as to open and cover an upper surface of the musical instrument main body ; and a cover lock portion formed in the upper surface of the musical instrument main body for locking the cover when the cover is opened and folded at the foldable portion . the hinge portion consists of a flexible synthetic resin which may be formed integrally with the cover or separately from the cover .