Programmable logic controller and base unit and module unit thereof

Provided is a programmable logic controller capable of lasting waterproofness or dust proofness. The base unit (1) has a substrate (10), a base housing (12) that accommodates the substrate (10), and a holding part (14) that holds a connection unit (2a), which is one of the plurality of module units. The connection unit (2a) has: a module-side connector (20); an engagement section (21) that is detachably engaged with the holding section (14); and a module housing (22) that has a fitting section that fits into the base housing (12). Furthermore, the programmable logic controller is provided with: an elastic member (31) that applies a force in a first direction, which is a direction in which the connection unit (2a) is separated from the base unit (1), and presses the engagement section of the connection unit (2a) against the holding section (14) of the base unit (1); and a sealing material (30) that seals a gap between the fitting parts of the base housing (12) and the module housing (22). 提供能够使防水性或防尘性持久的可编程逻辑控