Currently, the excel package in Flutter does not natively support embedding custom fonts, such as “ＭＳ Ｐゴシック,” into an Excel file. The package only works with the built-in fonts, and it lacks the functionality for specifying external fonts or embedding .ttf files directly in the generated Excel file.

However, you can consider a few alternative approaches to achieve this, like

• Use Post-Processing in Excel
• Use an External Library that Supports Custom Fonts
• Apply Font Styles with Excel Macros
• HTML to Excel Conversion

I found the solution to add custom font in the flutter excel generator by using “syncfusion_flutter_xlsio” sdk.

I have added the sample code for the reference

pubspeck.yaml file

 fonts:
- family: ＭＳ Ｐゴシック
  fonts:
    - asset: assets/font/MSPGothic.ttf

import ‘package:syncfusion_flutter_xlsio/xlsio.dart’ as xlsio;

generateXLSX() async {

final xlsio.Workbook workbook = xlsio.Workbook();
final xlsio.Worksheet sheet = workbook.worksheets[0];
sheet.enableSheetCalculations();

// Set the custom font
final xlsio.Style style = workbook.styles.add('customFontStyle');
style.fontName = "ＭＳ Ｐゴシック";
style.fontSize = 12;

// Apply style to a cell
  
  sheet.getRangeByIndex(1, 1).cellStyle = style;
  sheet.getRangeByIndex(1, 1).setText("益田市");


// Save the file
final List<int> bytes = workbook.saveAsStream();

final Uint8List data = Uint8List.fromList(bytes);

var value = await FileSaver.instance.saveFile(
  name: 'fileName.xlsx',
  bytes: data,
);

workbook.dispose();

}