A control device for an induction heating system for heating a material as it is conveyed through a conveyance channel of the induction heating system in a conveyance direction, the induction heating system comprising: a pair of master C-shaped heating units (5a, 5b) arranged along the conveyance direction on one side of the conveyance channel; and a pair of slave C-shaped heating units (6a, 6b) arranged along the conveyance direction on the other side of the conveyance channel opposite to the pair of master C-shaped heating units, wherein the pair of master C-shaped heating units (5a, 5b) and the pair of slave C-shaped heating units (6a, 6b) form a rectangular arrangement, the control device comprising: a master frequency control part arranged to set an operation frequency of a master inverter, which drives the pair of master C-shaped heating units (5a, 5b), so that a phase of an output voltage and a phase of an output current from the master inverter are synchronized; a slave frequency control part arranged to synchronize an operation frequency of a slave inverter, which drives the pair of slave C-shaped heating units (6a, 6b), with the operation frequency of the master inverter; a slave current phase control part arranged to synchronize a phase of an output current from the slave inverter with the phase of the output current from the master inverter; a master voltage control part arranged to set a pulse width of the output voltage from the master inverter; a slave voltage control part arranged to set a pulse width of an output voltage from the slave inverter; and a slave voltage phase control part arranged to synchronize a phase of the output voltage from the slave inverter with the phase of the output voltage from the master inverter after the slave current phase control part synchronises the phase of the output current from the slave inverter with the phase of the output current from the master inverter, wherein the master inverter and slave inverter are arranged to drive the pair of master C-shaped heating units (5a, 5b) and the pair of slave C-shaped heating units (6a, 6b) such that: directions of magnetic flux in the pair of master C-shaped heating units (5a, 5b) are opposite to each other, directions of magnetic flux in the pair of slave C-shaped heating units (6a, 6b) are opposite to each other, directions of magnetic flux in the respective ones (5a, 6a) of the pair of master C-shaped heating units (5a, 5b) and the pair of slave C-shaped heating units (6a, 6b) which are upstream along the conveyance direction are opposite to each other, and directions of magnetic flux in the respective ones (5b, 6b) of the pair of master C-shaped heating units (5a, 5b) and the pair of slave C-shaped heating units (6a, 6b) which are downstream along the conveyance direction are opposite to each other. a master frequency control part arranged to set an operation frequency of a master inverter, which drives the pair of master C-shaped heating units (5a, 5b), so that a phase of an output voltage and a phase of an output current from the master inverter are synchronized; a slave frequency control part arranged to synchronize an operation frequency of a slave inverter, which drives the pair of slave C-shaped heating units (6a, 6b), with the operation frequency of the master inverter; a slave current phase control part arranged to synchronize a phase of an output current from the slave inverter with the phase of the output current from the master inverter; a master voltage control part arranged to set a pulse width of the output voltage from the master inverter; a slave voltage control part arranged to set a pulse width of an output voltage from the slave inverter; and a slave voltage phase control part arranged to synchronize a phase of the output voltage from the slave inverter with the phase of the output voltage from the master inverter after the slave current phase control part synchronises the phase of the output current from the slave inverter with the phase of the output current from the master inverter, directions of magnetic flux in the pair of master C-shaped heating units (5a, 5b) are opposite to each other, directions of magnetic flux in the pair of slave C-shaped heating units (6a, 6b) are opposite to each other, directions of magnetic flux in the respective ones (5a, 6a) of the pair of master C-shaped heating units (5a, 5b) and the pair of slave C-shaped heating units (6a, 6b) which are upstream along the conveyance direction are opposite to each other, and directions of magnetic flux in the respective ones (5b, 6b) of the pair of master C-shaped heating units (5a, 5b) and the pair of slave C-shaped heating units (6a, 6b) which are downstream along the conveyance direction are opposite to each other.