PROBLEM TO BE SOLVED: To enable a stopping processing of a hybrid construction machine after turning an ignition key off.SOLUTION: This controller for the hybrid construction machine includes an internal power supply 260 for supplying power to a CPU 250, a first power supply line 210 and a second power supply line 230 for independently supplying power to the internal power supply 260, a switch 211 provided in the first power supply line 210 and adapted to be on/off when starting/stopping the hybrid construction machine, a voltage monitoring line 220 for inputting a voltage value for the first power supply line 210 to the CPU 250, and a semiconductor switch 232 provided in the second power supply line 230 and adapted to be controlled to be on/off by the CPU 250. The CPU 250 determines the on/off state of the switch 211 based on the voltage value for the first power supply line 210, and turns the semiconductor switch 232 on when determining that the switch 211 is on and also turns the semiconductor switch 232 on after determining that the switch 211 is off.