Casing of an electronic keyboard musical instrument includes two discrete parts: a main casing section; and a rear casing section. The main casing section includes a bottom plate having a keyboard section placed thereon, a keyslip portion extending upward from the bottom plate in front of the keyboard section, and left and right side plates extending upward from the bottom plate at left and right sides of the keyboard section. The rear casing section includes a roof plate forming a rear upper surface of the casing, and a back plate extending downward from a rear edge of the roof plate. The rear casing section is detachably attached to the main casing section by means of a slidingly-attaching structure for slidingly attaching the rear casing section to the left and right side plates of the main casing section.