A booster converter comprising: a rectifying circuit that rectifies voltage of a power supply; a reactor that is connected to an output side of the rectifying circuit; short-circuiting means that is constituted of a switching element connected to an output side of the reactor and controls at least one of electric current and voltage by switching of the switching element; one or more backflow preventing elements whose one end is connected to the output side of the reactor and that prevent electric current generated by switching of the short circuiting means from flowing backward from a load side toward the power supply side; and commutating means adapted to perform reverse recovery by applying low reverse bias voltage to the one or more backflow preventing elements, the commutating means being adapted to perform a commutation operation for causing electric current to flow toward a different electric-current path that is connected in parallel with an electric-current path including the one or more backflow preventing elements; control means adapted to control the commutating means to perform commutation operation immediately before turning on the short-circuiting means; the booster converter characterised by: malfunction detecting means that is installed in a path of a current flowing through the short-circuiting means or the backflow preventing elements and detects an electric current flowing through the short-circuiting means or the backflow preventing elements to detect a malfunction of the commutating means; and the control means is further adapted to perform operation control related to protection of the commutating means on a basis of detection by the malfunction detecting means, wherein the commutating means includes: a transformer having a primary winding and a secondary winding provided in the different electric-current path, the transformer performs the commutation operation by applying voltage based on voltage related to the primary winding to the secondary winding on the different electric-current path, a commutation rectifying element that rectifies the electric current flowing through the different electric-current path, a transformer driving unit that controls excitation current caused by application of voltage to the primary winding of the transformer, the transformer driving unit further comprising: a commutation power supply connected to the primary winding of the transformer, and a commutation switch that opens and closes to control the excitation current flowing from the commutation power supply toward the primary winding of the transformer based on a signal from the control means. a rectifying circuit that rectifies voltage of a power supply; a reactor that is connected to an output side of the rectifying circuit; short-circuiting means that is constituted of a switching element connected to an output side of the reactor and controls at least one of electric current and voltage by switching of the switching element; one or more backflow preventing elements whose one end is connected to the output side of the reactor and that prevent electric current generated by switching of the short circuiting means from flowing backward from a load side toward the power supply side; and commutating means adapted to perform reverse recovery by applying low reverse bias voltage to the one or more backflow preventing elements, the commutating means being adapted to perform a commutation operation for causing electric current to flow toward a different electric-current path that is connected in parallel with an electric-current path including the one or more backflow preventing elements; malfunction detecting means that is installed in a path of a current flowing through the short-circuiting means or the backflow preventing elements and detects an electric current flowing through the short-circuiting means or the backflow preventing elements to detect a malfunction of the commutating means; and the control means is further adapted to perform operation control related to protection of the commutating means on a basis of detection by the malfunction detecting means, a transformer having a primary winding and a secondary winding provided in the different electric-current path, the transformer performs the commutation operation by applying voltage based on voltage related to the primary winding to the secondary winding on the different electric-current path, a commutation rectifying element that rectifies the electric current flowing through the different electric-current path, a transformer driving unit that controls excitation current caused by application of voltage to the primary winding of the transformer, the transformer driving unit further comprising: a commutation power supply connected to the primary winding of the transformer, and a commutation switch that opens and closes to control the excitation current flowing from the commutation power supply toward the primary winding of the transformer based on a signal from the control means. a commutation power supply connected to the primary winding of the transformer, and a commutation switch that opens and closes to control the excitation current flowing from the commutation power supply toward the primary winding of the transformer based on a signal from the control means.