I want to convert japanese text to speech using microsoft speech sdk 5.1. I have found there 5 kinds of voices including simplified chinese. Chinese text to speech works fine.

foreach (ISpeechObjectToken Token in speech.GetVoices(string.Empty, string.Empty))
{
string str = Token.GetDescription(49).ToString();
if (str.Equals("Microsoft Simplified Chinese"))
{
speech.Voice = speech.GetVoices(string.Empty, string.Empty).Item(3);//Item(cmbVoices.SelectedIndex);
}
// Populate the ComboBox Entries ..
cmbVoices.Items.Add(Token.GetDescription(49));
}

In the documentation of Microsoft speech sdk I have found that the sdk supports Japanese text.

How can i convert Japanese text to speech?

The Phoneme set can be found on this page. You'll also find lots of pages on the links to the left (on that page) about using SAPI 5.3.