[Note]: Google Now does not work....

This issue was solved in Google now が使えなくなった（ONできない） (Japanese Google Search Help Forum) , so I'd like to note about how to solve it.
The error message is "Network error. Check your network connections".
But the Nexus 5 (Android 5.0.1) is connected on Wi-Fi.

Step 1. Clear "Clear data" in "Google Search" Apps.

Reference: Trying to start Google Now gives "Network error. Check your network connections (Google Nexus Help Forum)

• Settings --> Apps --> Google Search --> Clear Data

Step 2. Delete "Card Preferences" in Google Now.

Reference: Issue 14: Google Now Network Error (android-developer-preview)

• Settings --> Accounts --> Google --> Search --> Now Cards --> Delete card preferences. 

* Google app settings (Google Search Help)

Step 3. Logout Google Accounts

* There is a risk for your Account's data, please see "Remove an account" section in "Sign out (Android)" 

3rd March, 2015  @kimipooh