We propose a method of converting a real-weighted voting classifier to a compact integer-weighted voting classifier. Real-weighted voting classifiers like those trained using boosting are very popular and widely used due to their high prediction performance. Real numbers, however, are space-consuming and its floating-point arithmetic is slow compared to integer arithmetic, so compact integer weights are preferable for implementation on devices with small computational resources. Our conversion makes use of given feature vectors and solves an integer linear programming problem that minimizes the sum of integer weights under the constraint of keeping the classification result for the vectors unchanged. According to our experimental results using datasets of UCI Machine Learning Repository, the bit representation sizes are reduced to $5.2$-$33.4$% within $3.7$% test accuracy degrade in 7 of 8 datasets for the weighted voting classifiers of decision stumps learned using AdaBoost-SAMME.