A housing structure of an electronic keyboard musical instrument, in which a pivotable upper case can be mounted to and dismounted from a lower case when the upper case is in a semi-open state, thereby improving safety and the working efficiency at the time of assembly and maintenance. Pivot coupling parts each include an engagement protrusion comprised of a narrow-width portion and a wide-width portion. An upper case is formed with engagement holes each comprised of a narrow-width counterpart permitting the narrow-width portion to be inserted thereinto and a wide-width counterpart permitting the wide-width portion to be inserted thereinto. By moving the upper case, having a posture thereof corresponding to a mountable/dismountable angle range, in the direction in which the wide-width portions project, the wide-width portions can be inserted into and removed from the wide-width counterparts.