The number of farmers has decreased dramatically, and the average age of the famers is growing over 65 in Japan. Pumpkin is wildly planted in Hokkaido, because there is almost no requirement of spraying and weeding management after transplanting. Therefore, many farmers want to expand the production of pumpkins. However, it is a heavy job to harvest the pumpkins. An autonomous harvesting machine for the pumpkin is required to be developed. The objective of this study is to find out the fruit position of the pumpkin, which is the basic technology for the auto-harvesting machine. Because the colour of the pumpkin fruits is similar to their leaves, the position of the fruits is difficult to be detected directly. In fact, the position of the pumpkin fruits is similar with the position of the flowers. Because the colour of the flowers and leaves are near yellow and green, respectively, the flowers could be detected by an image processing method. In this study, a colour camera was utilized to capture the images at an outdoor field. The colour space of the image was transformed from red, green and blue (RGB) to hue, saturation and value (HSV). The yellow flowers were detected in the HSV colour space. An experiment was conducted at an outdoor field of pumpkin in Hokkaido, Japan at daytime and evening. The results show that the method can detect out the flower correctly at both conditions, though some errors occur at the conditions when the colour of the background was similar with the flowers, or the colour of the flowers became white when the flowers were withered.