Provided are a watertight plate where a resin is prevented from leaking and a power supply device including the same. In a base plate (watertight plate) of the present invention, when demarcating a channel by allowing a channel sidewall to contact with a cover plate, a resin is applied onto a second step portion of the channel sidewall to improve watertightness of the channel. The second step portion is closer to the channel and lower than a first step portion, which is a step portion in contact with the cover plate. Therefore, the movement of the resin applied onto the second step portion is prohibited by an edge surface of the step portion higher than the second step portion. Moreover, a resin lump of the resin applied onto the second step portion is housed within a third step portion which is closer to the channel and lower than the second step portion.