1. A body cavity observation apparatus comprising: a treatment tool to be inserted into a body cavity of a subject with an insertion tool as a guide, an observation section attached to an opening of a body wall of the subject, a monitor for displaying the body cavity captured by the observation section, and a marker position-detecting device for detecting a position of a marker part applied to the treatment tool or insertion tool from an image showing the body cavity captured by the observation section; wherein the marker position-detecting device comprises: a pixel extracting device for extracting a group of pixels in the same color as color of the marker part from the image, and a calculation device for calculating a position of the barycenter of the group of pixels as a position of the marker part; wherein the pixel extracting device extracts a pixel whose output value of a color component included in a color of the marker part among R, G, and B output values outputted from an image pickup device of the observation section is bigger than an output value of a color component that is not included in a color of the marker part by a predetermined threshold of the group of pixels.