A method of controlling a polyphase load in a driving device, the driving device comprising: first to M th output lines (L1 L2, L3) connected to a M-phase load; first to N th input lines (L4, L5, L6) connected to an N-phase power supply (R, S, T); a group of switches interposed in said first to N th input lines and allow said first input line to be conductive independently of whether said second to N th input lines are conductive or nonconductive; a first DC power line; a second DC power line to which a potential is applied that is lower than a potential applied to said first DC power line; a diode bridge for rectifying an N-phase alternating current applied to said first to N th input lines to apply a DC voltage between said first DC power line and said second DC power line; first to M th high arm side switches (Q11, Q12, Q13) connected respectively between said first DC power line and said first to M th output lines; and first to M th low arm side switches (Q21, Q22, Q23) connected respectively between said second DC power line and said first to M th output lines, the method being characterized by: (a) a step of making said first input line conductive while said second to N th input lines are kept nonconductive in said group of switches; (b) a step (102; 201, 204, 206) of making at least one of said first to M th low arm side switches conductive; and (c) a step (103; 202) of determining whether or not a current flowing through said first input line, said at least one of said first to Mth low arm side switches and said second DC power line exceeds a first predetermined value. first to M th output lines (L1 L2, L3) connected to a M-phase load; first to N th input lines (L4, L5, L6) connected to an N-phase power supply (R, S, T); a group of switches interposed in said first to N th input lines and allow said first input line to be conductive independently of whether said second to N th input lines are conductive or nonconductive; a first DC power line; a second DC power line to which a potential is applied that is lower than a potential applied to said first DC power line; a diode bridge for rectifying an N-phase alternating current applied to said first to N th input lines to apply a DC voltage between said first DC power line and said second DC power line; first to M th high arm side switches (Q11, Q12, Q13) connected respectively between said first DC power line and said first to M th output lines; and first to M th low arm side switches (Q21, Q22, Q23) connected respectively between said second DC power line and said first to M th output lines, the method being characterized by: (a) a step of making said first input line conductive while said second to N th input lines are kept nonconductive in said group of switches; (b) a step (102; 201, 204, 206) of making at least one of said first to M th low arm side switches conductive; and (c) a step (103; 202) of determining whether or not a current flowing through said first input line, said at least one of said first to Mth low arm side switches and said second DC power line exceeds a first predetermined value. (a) a step of making said first input line conductive while said second to N th input lines are kept nonconductive in said group of switches; (b) a step (102; 201, 204, 206) of making at least one of said first to M th low arm side switches conductive; and (c) a step (103; 202) of determining whether or not a current flowing through said first input line, said at least one of said first to Mth low arm side switches and said second DC power line exceeds a first predetermined value.