To prevent charged material from directly hitting a sealing member, by arranging an opening or closing function and a function of press fitting the sealing member for higher sealability while a moving track of a gate is held in a smaller space.

A top part opening/closing gate apparatus 30 of a stock preheating tank comprises a hopper 19 which is provided at a top part of the stock preheating tank 4 and has a horizontal open lower end, an elastic sealing member 22 provided along the outer circumference of the open lower end of the hopper 19, a gate 16 which has the top surface of the outer circumference part thereof contacting the sealing member and a opening/closing device 32 for open or close the gate. The opening/closing device 32 is provided with a hoisting drive shaft 17 and a traversing drive shaft 18 which are arranged parallel with each other and separated at an interval across the width thereof, a hoisting link mechanism 34 to move the gate between a position U of pressing onto the sealing member and a position D of isolating therefrom by oscillating the hoisting drive shaft, and a traversing link mechanism 36 to move the gate between the isolation position D and an opening position O by oscillating the traversing drive shaft.