An environment control system comprising: a control unit; a serial communication bus including a main communication line having a first end connected to the control unit, and first to Nth (N: an integer of 2 or more) sub communication lines (SL1 to SL3) branched off from a second end of the main communication line; devices (21 to 26) connected to the first to Nth sub communication lines (SL1 to SL3); and switching units (SW2-1, SW2-2, SW3-1, SW3-2) disposed on the sub communication lines (SL2, SL3) excluding the first sub communication line, and configured to connect and disconnect the devices (24 to 26) to and from the control unit, each of the first to Nth sub communication lines (SL1 to SL3) being connected to one or more of the devices, wherein when the control unit abnormally communicates with the devices (21 to 26), the control unit opens the switching units (SW2-1, SW2-2, SW3-1, SW3-2), and determines, when the control unit abnormally communicates with a device or devices (21 to 23) connected to the first sub communication line, that a malfunction occurs at the device or devices (21 to 23) connected to the first sub communication line, and when the control unit abnormally communicates with the devices (21 to 26), the control unit opens the switching units (SW2-1, SW2-2, SW3-1, SW3-2), and if the control unit normally communicates with the device or devices (21 to 23) connected to the first sub communication line, the control unit closes the switching units one by one in sequence, and when the control unit abnormally communicates with a device or devices connected to a sub communication line (SL2, SL3) for which the switching unit is closed, the control unit determines that a malfunction occurs at the device or devices (24 to 26). a control unit; a serial communication bus including a main communication line having a first end connected to the control unit, and first to Nth (N: an integer of 2 or more) sub communication lines (SL1 to SL3) branched off from a second end of the main communication line; a main communication line having a first end connected to the control unit, and first to Nth (N: an integer of 2 or more) sub communication lines (SL1 to SL3) branched off from a second end of the main communication line; devices (21 to 26) connected to the first to Nth sub communication lines (SL1 to SL3); and switching units (SW2-1, SW2-2, SW3-1, SW3-2) disposed on the sub communication lines (SL2, SL3) excluding the first sub communication line, and configured to connect and disconnect the devices (24 to 26) to and from the control unit, each of the first to Nth sub communication lines (SL1 to SL3) being connected to one or more of the devices, wherein when the control unit abnormally communicates with the devices (21 to 26), the control unit opens the switching units (SW2-1, SW2-2, SW3-1, SW3-2), and determines, when the control unit abnormally communicates with a device or devices (21 to 23) connected to the first sub communication line, that a malfunction occurs at the device or devices (21 to 23) connected to the first sub communication line, and when the control unit abnormally communicates with the devices (21 to 26), the control unit opens the switching units (SW2-1, SW2-2, SW3-1, SW3-2), and if the control unit normally communicates with the device or devices (21 to 23) connected to the first sub communication line, the control unit closes the switching units one by one in sequence, and when the control unit abnormally communicates with a device or devices connected to a sub communication line (SL2, SL3) for which the switching unit is closed, the control unit determines that a malfunction occurs at the device or devices (24 to 26).