Well, the number of workers was 1.
I mean the i-th iteration of for-loop in the code below generates the same random value.
I created data-set once, created data-loader at every epoch.
When I used random.*, no problem, but I had the issue when I used numpy.random.*.
for epoch in range(100):
    kwargs = {'num_workers': 1, 'pin_memory': True} if args.cuda else {}
    train_loader = torch.utils.data.DataLoader(
        dataset('../data', train=True, download=True, transform=transform),
        batch_size=args.batch_size, shuffle=True, **kwargs)
    #
    for data in enumerate(train_loader):
        do_something()