A bug report is a document indicating when a bug occurs. Developers discuss and resolve the bug through comments in the report. The time required to fix a bug can depend on the bug report. Although many studies have researched bug reports, few have examined bug report comments. Herein we adopt a convolutional neural network (CNN), which is a class of deep neural networks, to classify bug reports into those with short and long fixing times based on the data collected from a bug tracking system. Then we extract the features related to the bug fixing time by visualizing the decision basis that the CNN model uses in the prediction process. We employ a gradient-based visualization technique called Grad-cam to visualize the word sequence that the CNN model uses in the prediction. We use the top ten word sequences as the decision basis to extract the features of the bug report. An experiment confirmed that our method classified more than 36, 000 actual bug reports taken from Bugzilla by short and long fixing times with 75-80% accuracy. Further visualization using Grad-cam shows the difference in the stack trace and the degree of abstraction of the words used. Bug reports with a short bug fixing time are specific and informative with regard to stack trace descriptions. In contrast, those with a long bug fixing time are abstract.