gulp watch実行時に「Error: Cannot find module ‘gulp-sass’」エラーが発生した際の対処方法

> gulp watch
Error: Cannot find module 'gulp-sass'

Require stack:
- \gulpfile.js
- C:\Users\user\AppData\Roaming\npm\node_modules\gulp-cli\lib\shared\require-or-import.js
- C:\Users\user\AppData\Roaming\npm\node_modules\gulp-cli\lib\versioned\^4.0.0\index.js
- C:\Users\user\AppData\Roaming\npm\node_modules\gulp-cli\index.js
- C:\Users\user\AppData\Roaming\npm\node_modules\gulp-cli\bin\gulp.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:880:15)
    at Function.Module._load (internal/modules/cjs/loader.js:725:27)
    at Module.require (internal/modules/cjs/loader.js:952:19)
    at require (internal/modules/cjs/helpers.js:88:18)
    at Object.<anonymous> (gulpfile.js:4:12)
    at Module._compile (internal/modules/cjs/loader.js:1063:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1092:10)
    at Module.load (internal/modules/cjs/loader.js:928:32)
    at Function.Module._load (internal/modules/cjs/loader.js:769:14)
    at Module.require (internal/modules/cjs/loader.js:952:19) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '\\gulpfile.js',
    'C:\\Users\\user\\AppData\\Roaming\\npm\\node_modules\\gulp-cli\\lib\\shared\\require-or-import.js',
    'C:\\Users\\user\\AppData\\Roaming\\npm\\node_modules\\gulp-cli\\lib\\versioned\\^4.0.0\\index.js',
    'C:\\Users\\user\\AppData\\Roaming\\npm\\node_modules\\gulp-cli\\index.js',
    'C:\\Users\\user\\AppData\\Roaming\\npm\\node_modules\\gulp-cli\\bin\\gulp.js'
  ]
}

>  npm install gulp-sass --save-dev

> gulp watch
 Using gulpfile gulpfile.js
Starting 'watch'...