A brightness extraction circuit 44a in the feature amount comparison part 44 to which an imaging image (an endoscope image) based on an imaging part of an endoscope is inputted calculates a difference between a maximum value and a minimum value of a luminance average value in a divided area of the imaging image, and the comparison circuit 44B is whether a difference is less than a threshold or not The trigger signal generator 45 generates a trigger signal when the threshold value is less than or equal to the threshold value, and the three-dimensional model image generating part 41a starts the processing of constructing the three-dimensional shape image from the imaging image and position information based on the sensor 34 in the tip part of the endoscope.内視鏡の撮像部に基づく撮像画像(内視鏡画像)が入力される特徴量比較部44内の輝度抽出回路44aは、撮像画像の分割領域の輝度平均値の最大値と最小値との差分を算出し、比較回路44bは差分が閾値以下か否かを比較し、閾値以下の場合にはトリガ信号生成部45はトリガ信号を生成し、3次元モデル画像生成部41aは、撮像画像と、内視鏡の先端部内センサ34に基づく位置情報から3次元形状画像を構築する処理を開始する。
