A cutter includes a handle, a blade unit and a lock. The handle includes a slot in an end. The blade unit includes a holder and a blade. The holder includes an insert extending from a sheath. The sheath includes two channels made with small width so that a finger cannot be inserted into any of the channels. The insert is adapted for insertion in the slot. The blade is inserted in the sheath, with two exposed portions exposed to the exterior of the sheath through the channels. The lock is connected to the handle near the slot and movable between a locking position for keeping the insert in the slot and a releasing position for allowing the insert to leave the slot.