A wall-mounted housing apparatus, which is configured to house a component of an electronic apparatus and is to be installed on a wall surface, the wall-mounted housing apparatus installed on the wall surface having defined therein an apparatus width direction and an apparatus height direction that are parallel to the wall surface, and an apparatus depth direction that is perpendicular to the wall surface, the wall-mounted housing apparatus comprising: a rear case having a front side opening portion and having a semi-container shape, the rear case being configured to house the component of the electronic apparatus, the rear case having a back surface that is mounted to the wall surface; a front cover having a plate shape, the front cover being mounted to an upper side of the front side opening portion of the rear case through intermediation of a hinge mechanism so as to be openable and closable upward at an angle larger than 90°; and a retaining mechanism interposed between the rear case and the front cover, for retaining the front cover in a state of being opened at 90°, the retaining mechanism comprising: a fixed engagement portion provided to the upper side of the front side opening portion of the rear case; and a movable engagement portion provided to a back surface of the front cover at a position corresponding to the fixed engagement portion, for retaining the front cover in the state of being opened at 90° in such a manner that the movable engagement portion in a state in which the front cover is opened at 90° disengageably engages with the fixed engagement portion, the fixed engagement portion having a support surface facing upward, the movable engagement portion comprising a slide bar slidable in the apparatus height direction which is the direction when the front cover is closed, the slide bar comprising: a bar main body slidable in the apparatus height direction; and a portion to be supported, which is projected downward in the apparatus height direction from an end portion of the bar main body, which is located on a depth side in the apparatus depth direction when the front cover is opened upward at 90°, the front cover being retained in the state of being opened at 90° in such a manner that, when the front cover is opened upward at the angle larger than 90°, the slide bar slides toward the depth side in the apparatus depth direction due to its weight, and then, when the front cover is closed at 90°, the portion to be supported is supported by the support surface of the fixed engagement portion. a rear case having a front side opening portion and having a semi-container shape, the rear case being configured to house the component of the electronic apparatus, the rear case having a back surface that is mounted to the wall surface; a front cover having a plate shape, the front cover being mounted to an upper side of the front side opening portion of the rear case through intermediation of a hinge mechanism so as to be openable and closable upward at an angle larger than 90°; and a retaining mechanism interposed between the rear case and the front cover, for retaining the front cover in a state of being opened at 90°, the retaining mechanism comprising: a fixed engagement portion provided to the upper side of the front side opening portion of the rear case; and a movable engagement portion provided to a back surface of the front cover at a position corresponding to the fixed engagement portion, for retaining the front cover in the state of being opened at 90° in such a manner that the movable engagement portion in a state in which the front cover is opened at 90° disengageably engages with the fixed engagement portion, a fixed engagement portion provided to the upper side of the front side opening portion of the rear case; and a movable engagement portion provided to a back surface of the front cover at a position corresponding to the fixed engagement portion, for retaining the front cover in the state of being opened at 90° in such a manner that the movable engagement portion in a state in which the front cover is opened at 90° disengageably engages with the fixed engagement portion, the fixed engagement portion having a support surface facing upward, the movable engagement portion comprising a slide bar slidable in the apparatus height direction which is the direction when the front cover is closed, the slide bar comprising: a bar main body slidable in the apparatus height direction; and a portion to be supported, which is projected downward in the apparatus height direction from an end portion of the bar main body, which is located on a depth side in the apparatus depth direction when the front cover is opened upward at 90°, a bar main body slidable in the apparatus height direction; and a portion to be supported, which is projected downward in the apparatus height direction from an end portion of the bar main body, which is located on a depth side in the apparatus depth direction when the front cover is opened upward at 90°, the front cover being retained in the state of being opened at 90° in such a manner that, when the front cover is opened upward at the angle larger than 90°, the slide bar slides toward the depth side in the apparatus depth direction due to its weight, and then, when the front cover is closed at 90°, the portion to be supported is supported by the support surface of the fixed engagement portion.