A power supply device includes a power supply input portion, a rectification portion, a smoothing portion, a power conversion portion, a power supply output portion, a signal input portion, a control portion, a circuit substrate, and a case. The circuit substrate is formed in an elongated rectangular plate-like shape. The power supply input portion is mounted on a first end portion of the circuit substrate in a longitudinal direction. The rectification portion, the smoothing portion, the power conversion portion, the control portion, and the power supply output portion are mounted on the circuit substrate in the stated order from the first end portion toward a second end portion in the longitudinal direction. The signal input portion is mounted at a position closer to the second end portion than the rectification portion in the circuit substrate.