A side trimming apparatus comprising: a cutting unit which is installed on at least one side of a steel plate being transferred and trims a side of the steel plate through mechanical cutting; and a laser processing unit which is installed in front of the cutting unit and emits a laser beam to a portion of the steel plate to be cut to form a groove at the portion to be cut or preheat the portion to be cut, wherein the cutting unit comprises: a stand; an upper rotatable knife installed at the stand and disposed above the steel plate; and a lower rotatable knife installed at the stand and disposed below the steel plate to apply a shearing force to the side of the steel plate together with the upper knife, wherein the laser processing unit comprises: a laser oscillator which generates a laser beam; and an optical condensing head which is installed at the stand and focuses the laser beam of the laser oscillator on the steel plate, wherein the optical condensing head is installed at the stand to be rotatably driven to adjust an emission position of the laser beam, characterised in that the side trimming apparatus further comprises a controller which is connected to the laser processing unit and controls a laser output of the laser processing unit and an input portion which is connected to the controller and receives at least one of transfer speed information and steel grade information of the steel plate input from a main controller of a cold rolling line, wherein the controller controls the output of the laser processing unit based on the input information of the input portion, and wherein the controller adjusts the rotation angle of the condensing head according to the transfer speed of the steel plate to adjust the distance (D1, D2) from the mechanical cutting point to the laser irradiation position, and controls the laser processing unit to adjust a spot shape or size of the laser beam. a cutting unit which is installed on at least one side of a steel plate being transferred and trims a side of the steel plate through mechanical cutting; and a laser processing unit which is installed in front of the cutting unit and emits a laser beam to a portion of the steel plate to be cut to form a groove at the portion to be cut or preheat the portion to be cut, wherein the cutting unit comprises: a stand; an upper rotatable knife installed at the stand and disposed above the steel plate; and a lower rotatable knife installed at the stand and disposed below the steel plate to apply a shearing force to the side of the steel plate together with the upper knife, wherein the laser processing unit comprises: a stand; an upper rotatable knife installed at the stand and disposed above the steel plate; and a lower rotatable knife installed at the stand and disposed below the steel plate to apply a shearing force to the side of the steel plate together with the upper knife, wherein the laser processing unit comprises: a laser oscillator which generates a laser beam; and an optical condensing head which is installed at the stand and focuses the laser beam of the laser oscillator on the steel plate, wherein the optical condensing head is installed at the stand to be rotatably driven to adjust an emission position of the laser beam, characterised in that the side trimming apparatus further comprises a controller which is connected to the laser processing unit and controls a laser output of the laser processing unit and an input portion which is connected to the controller and receives at least one of transfer speed information and steel grade information of the steel plate input from a main controller of a cold rolling line, wherein the controller controls the output of the laser processing unit based on the input information of the input portion, and wherein the controller adjusts the rotation angle of the condensing head according to the transfer speed of the steel plate to adjust the distance (D1, D2) from the mechanical cutting point to the laser irradiation position, and controls the laser processing unit to adjust a spot shape or size of the laser beam.