how to let the log only log ERROR level information

I tried lots of way to use the ERROR level, but still all information was wrote into log file

python 3.5 + scrapy 1.2

``` python
dmoz_spider.py

configure_logging(install_root_handler=False)
logging.basicConfig(filename='log.txt', format='%(levelname)s: %(message)s', level=logging.ERROR)
```

``` python
settings.py

...................
LOG_LEVEL = "ERROR"
```
