A double-balloon endoscope system comprising: an endoscope including a flexible insertion section to be inserted into a body cavity, a plurality of magnetic-field generating elements provided in the insertion section, a first balloon disposed on the outer surface of the distal end of the insertion section, a second balloon disposed on the outer surface of the distal end of overtube means through which the insertion section is to be inserted, a magnetic-field generating element provided at a known position at the distal end of the overtube means, a magnetic-field detecting element provided at a known position in the vicinity of a subject into which the insertion section is to be inserted, the magnetic-field detecting element being configured to detect a first magnetic field generated from each of the plurality of magnetic-field generating elements provided in the insertion section and a second magnetic field generated from the magnetic-field generating element provided at the distal end of the overtube means; balloon control means configured to control the first and second balloons (118, 122) to inflate and/or deflate; characterised by further comprising: a state-image generating means configured to estimate, based on the first magnetic field and the second magnetic field detected by the magnetic-field detecting element, an insertion shape of the insertion section inserted into the body cavity to generate an insertion shape image and estimate a position of the first balloon and a position of the second balloon with respect to the insertion section, and, based on inflation/deflation state information indicating inflation/deflation states of the first balloon and the second balloon by control of the balloon control means, superimpose a first marker at a position of the first balloon in the insertion shape image and superimpose a second marker at a position of the second balloon in the insertion shape image. an endoscope including a flexible insertion section to be inserted into a body cavity, a plurality of magnetic-field generating elements provided in the insertion section, a first balloon disposed on the outer surface of the distal end of the insertion section, a second balloon disposed on the outer surface of the distal end of overtube means through which the insertion section is to be inserted, a magnetic-field generating element provided at a known position at the distal end of the overtube means, a magnetic-field detecting element provided at a known position in the vicinity of a subject into which the insertion section is to be inserted, the magnetic-field detecting element being configured to detect a first magnetic field generated from each of the plurality of magnetic-field generating elements provided in the insertion section and a second magnetic field generated from the magnetic-field generating element provided at the distal end of the overtube means; balloon control means configured to control the first and second balloons (118, 122) to inflate and/or deflate; characterised by further comprising: a state-image generating means configured to estimate, based on the first magnetic field and the second magnetic field detected by the magnetic-field detecting element, an insertion shape of the insertion section inserted into the body cavity to generate an insertion shape image and estimate a position of the first balloon and a position of the second balloon with respect to the insertion section, and, based on inflation/deflation state information indicating inflation/deflation states of the first balloon and the second balloon by control of the balloon control means, superimpose a first marker at a position of the first balloon in the insertion shape image and superimpose a second marker at a position of the second balloon in the insertion shape image. a state-image generating means configured to estimate, based on the first magnetic field and the second magnetic field detected by the magnetic-field detecting element, an insertion shape of the insertion section inserted into the body cavity to generate an insertion shape image and estimate a position of the first balloon and a position of the second balloon with respect to the insertion section, and, based on inflation/deflation state information indicating inflation/deflation states of the first balloon and the second balloon by control of the balloon control means, superimpose a first marker at a position of the first balloon in the insertion shape image and superimpose a second marker at a position of the second balloon in the insertion shape image.