A programmable controller for controlling a plant on the basis of a stored program includes a processing unit (1) for reading out an instruction from a program memory (13) to process the instruction. In the processing unit, it is determined whether the instruction read out from the memory (13) is an instruction taking a register-modified addressing mode or an instruction taking a direct addressing mode. When the read-out instruction is an instruction taking the direct addressing mode, a plant is directly addressed by the address part of the instruction. Then, an arithmetic operation is performed using the state information which is read out from the plant thus addressed, or plant control information is sent from the processing unit to the plant thus addressed.