An endoscopic surgery device comprising: a guide member configured to penetrate through a body wall to be inserted into a body cavity; an endoscope insertion path which is provided inside the guide member, and into which an endoscope configured to observe an inside of the body cavity is insertable in a back-and-forth movable manner; and a treatment tool insertion path which is provided inside the guide member, and into which a treatment tool configured to inspect or treat a diseased part in the body cavity is insertable in a back-and-forth movable manner; characterized by detection means including a non-sensitive area where no change in a relative position of the treatment tool with respect to the endoscope is detected even if the treatment tool inserted into the treatment tool insertion path is moved back and forth, and a sensitive area which is an area other than the non-sensitive area, in which change in a relative position of the treatment tool is detected when the treatment tool is moved back and forth, the detection means configured to detect a movement amount of the treatment tool with respect to the guide member in the sensitive area; control means (314, 130) configured to change a range of an observation image acquired by the endoscope in accordance with the movement amount of the treatment tool detected by the detection means, and an interlocking member configured to be movable inside the treatment tool insertion path in interlock with the back-and-forth movement of the treatment tool, the interlocking member being provided with an allowance part corresponding to the non-sensitive area, beyond which the interlocking member starts moving in interlock with the back-and-forth movement of the treatment tool, wherein the detection means detects the movement amount of the treatment tool by detecting a movement amount of the interlocking member. a guide member configured to penetrate through a body wall to be inserted into a body cavity; an endoscope insertion path which is provided inside the guide member, and into which an endoscope configured to observe an inside of the body cavity is insertable in a back-and-forth movable manner; and a treatment tool insertion path which is provided inside the guide member, and into which a treatment tool configured to inspect or treat a diseased part in the body cavity is insertable in a back-and-forth movable manner; detection means including a non-sensitive area where no change in a relative position of the treatment tool with respect to the endoscope is detected even if the treatment tool inserted into the treatment tool insertion path is moved back and forth, and a sensitive area which is an area other than the non-sensitive area, in which change in a relative position of the treatment tool is detected when the treatment tool is moved back and forth, the detection means configured to detect a movement amount of the treatment tool with respect to the guide member in the sensitive area; control means (314, 130) configured to change a range of an observation image acquired by the endoscope in accordance with the movement amount of the treatment tool detected by the detection means, and an interlocking member configured to be movable inside the treatment tool insertion path in interlock with the back-and-forth movement of the treatment tool, the interlocking member being provided with an allowance part corresponding to the non-sensitive area, beyond which the interlocking member starts moving in interlock with the back-and-forth movement of the treatment tool, wherein the detection means detects the movement amount of the treatment tool by detecting a movement amount of the interlocking member.