PURPOSE: To enable stable transfer of a large-sized fish body having a body weight of about 2-12kg in the slicing of the body into three pieces along the backbone and to prevent the slip of the fish relative to the transferring conveyor caused by the cutting resistance applied by a rotary blade.

CONSTITUTION: A fish is gutted by opening the abdomen and the head is removed from the fish body. The body A is extended between a pair of horizontal main guide plates 21. A pair of clamping conveyors 22 are placed at both sides of the main guide plates and a pair of rotary saw blades 35 for cutting the back of the fish body are placed above the conveyors. At least one transfer rotary gear 31 is placed between the main guide plates 21 and the fish body Ag extending over the main guide plates 21 is engaged with the front and rear rotary gear 31 and forcedly transferred by the action of the gear in combination with the clamping power of the conveyors 22.