A package open knife to cut and open a package that houses containment, comprising: a main body, a blade portion attached to the main body, a grip portion provided with the main body to elongatedly extend in a predetermined first direction and a guide portion, the guide portion having a guide face to guide the movement of the blade portion by sliding in a face extending direction of the package in a state to face-contact with the package during the cutting operation of the package by the user holding the grip portion, wherein both the predetermined cutting depth and the predetermined cutting angle (θ) of the blade portion to the package are maintained when the guide face is in face-contact with the package, wherein the blade portion is embedded to the main body to project from the front side of the main body by the predetermined amount obliquely to the first direction and the blade portion includes a blade edge to cut the package in the draw-cutting direction defined by a direction from the blade portion to the grip portion, wherein, when the face extending direction of the guide face is defined as a second direction, the second direction is provided to be parallel with the first direction, the grip portion is provided with a length in the first direction to correspond to the entire length of a palm of the user holding the grip portion, the grip portion having a rib portion at the outer edge of the grip portion and a plate-like concave portion provided at the inside of the rib portion, the concave portion includes a finger placing face on which at least the thumb and the index finger of the user holding the grip portion can be placed, the grip portion includes finger engaging portions (71, 72, 73) in a direction to cross the draw-cutting direction to prohibit the finger of the user from moving in the draw-cutting direction during the draw cutting operation and to apply the finger force of the user holding the grip portion during the cutting operation in the draw-cutting direction, the finger engaging portions include, at the front side of the grip portion, an inclined engaging portion obliquely inclined to the first direction to engage with the thumb or index finger of the user and a concave engaging portion to dent in a direction to cross the first direction to engage with the index finger or the middle finger of the user, and at the rear side of the grip portion, a projected engaging portion projecting in a direction to cross the first direction to engage with the ring finger or the little finger of the user. the guide portion having a guide face to guide the movement of the blade portion by sliding in a face extending direction of the package in a state to face-contact with the package during the cutting operation of the package by the user holding the grip portion, wherein both the predetermined cutting depth and the predetermined cutting angle (θ) of the blade portion to the package are maintained when the guide face is in face-contact with the package, wherein the blade portion is embedded to the main body to project from the front side of the main body by the predetermined amount obliquely to the first direction and the blade portion includes a blade edge to cut the package in the draw-cutting direction defined by a direction from the blade portion to the grip portion, wherein, when the face extending direction of the guide face is defined as a second direction, the second direction is provided to be parallel with the first direction, the grip portion is provided with a length in the first direction to correspond to the entire length of a palm of the user holding the grip portion, the grip portion having a rib portion at the outer edge of the grip portion and a plate-like concave portion provided at the inside of the rib portion, the concave portion includes a finger placing face on which at least the thumb and the index finger of the user holding the grip portion can be placed, the grip portion includes finger engaging portions (71, 72, 73) in a direction to cross the draw-cutting direction to prohibit the finger of the user from moving in the draw-cutting direction during the draw cutting operation and to apply the finger force of the user holding the grip portion during the cutting operation in the draw-cutting direction, the finger engaging portions include, at the front side of the grip portion, an inclined engaging portion obliquely inclined to the first direction to engage with the thumb or index finger of the user and a concave engaging portion to dent in a direction to cross the first direction to engage with the index finger or the middle finger of the user, and at the rear side of the grip portion, a projected engaging portion projecting in a direction to cross the first direction to engage with the ring finger or the little finger of the user.