An outdoor unit of an air-conditioning apparatus, the outdoor unit comprising: a housing in which a machine chamber is formed; an electrical component box disposed in the machine chamber; and a power supply panel including a terminal block to which a connection wire is attached, the connection wire being connected to an electrical component held in the electrical component box, the power supply panel including a first power-supply-panel-side engagement member, and the electrical component box including an electrical-component-box-side engagement member configured to be engageable with and disengageable from the first power-supply-panel-side engagement member, wherein the first power-supply-panel-side engagement member and the electrical-component-box-side engagement member are configured to be engageable with and disengageable from each other by relative movement between each other, wherein the power supply panel is configured to be fixable to the housing in a state wherein the electrical-component-box-side engagement member and the first power-supply-panel-side engagement member are disengaged from each other, such that the power supply panel is positioned inside an outer periphery of the housing, as the housing is seen in a plan view, and the power supply panel is positioned below the electrical component box, characterised in that the power supply panel includes a rib, wherein the electrical component box includes an electrical-component-box-side slide surface, a first stopper, a second stopper, and a fixing piece, wherein the second stopper is positioned more to a side defined by a first direction than the first stopper, wherein the electrical-component-box-side slide surface is positioned more to a side defined by a second direction than the fixing piece, wherein in a state in which the first power-supply-panel-side engagement member and the electrical-component-box-side engagement member are engaged with each other, the power supply panel is slidable over the electrical-component-box-side slide surface by a reference distance in the first direction and the rib passes over the first stopper, and wherein, in this state, movement of the power supply panel in a direction opposite to the first direction is restrained by the first stopper, movement of the power supply panel in the first direction is restrained by the second stopper, movement of the power supply panel in a direction opposite to the second direction is restrained by the fixing piece, and movement of the power supply panel in the second direction is restrained by the electrical-component-box-side engagement member. a housing in which a machine chamber is formed; an electrical component box disposed in the machine chamber; and a power supply panel including a terminal block to which a connection wire is attached, the connection wire being connected to an electrical component held in the electrical component box, the power supply panel including a first power-supply-panel-side engagement member, and the electrical component box including an electrical-component-box-side engagement member configured to be engageable with and disengageable from the first power-supply-panel-side engagement member, wherein the first power-supply-panel-side engagement member and the electrical-component-box-side engagement member are configured to be engageable with and disengageable from each other by relative movement between each other, wherein the power supply panel is configured to be fixable to the housing in a state wherein the electrical-component-box-side engagement member and the first power-supply-panel-side engagement member are disengaged from each other, such that the power supply panel is positioned inside an outer periphery of the housing, as the housing is seen in a plan view, and the power supply panel is positioned below the electrical component box, characterised in that the power supply panel includes a rib, wherein the electrical component box includes an electrical-component-box-side slide surface, a first stopper, a second stopper, and a fixing piece, wherein the second stopper is positioned more to a side defined by a first direction than the first stopper, wherein the electrical-component-box-side slide surface is positioned more to a side defined by a second direction than the fixing piece, wherein in a state in which the first power-supply-panel-side engagement member and the electrical-component-box-side engagement member are engaged with each other, the power supply panel is slidable over the electrical-component-box-side slide surface by a reference distance in the first direction and the rib passes over the first stopper, and wherein, in this state, movement of the power supply panel in a direction opposite to the first direction is restrained by the first stopper, movement of the power supply panel in the first direction is restrained by the second stopper, movement of the power supply panel in a direction opposite to the second direction is restrained by the fixing piece, and movement of the power supply panel in the second direction is restrained by the electrical-component-box-side engagement member.