A power-supply monitoring device includes a power-supply control section, a detection section, a determination section, a control section, and an attachment section. The power-supply control section receives supply of power from a mains power source section, and selectively sets a power supply mode and a power saving mode. The detection section detects, in a predetermined region, a body capable of movement including a user who intends to use a processing section. When the body capable of movement is detected, the determination section determines whether switching from the power saving mode to the power supply mode is to be performed to obtain a result. The control section controls the power-supply control section on the basis of the result. The attachment section is attached to the detection section so that a detection region including a position or having an area for identifying the user is adjustable.