To obtain a positive type photoresist composition excellent in shelf stability and giving a resist pattern free from development defects.

The positive type photoresist composition contains an acid decomposable polysiloxane containing a structural unit having a naphthalene skeleton or an alkali-soluble polysiloxane containing a structural unit having a naphthalene sleketon, a photo-acid generator and a phenolic compound in which at least part of phenolic hydroxyl groups contained in its molecule is protected with an acid decomposable group or an aromatic or aliphatic carboxylic acid compound in which at least part of carboxyl groups contained in its molecule is protected with an acid decomposable group.