Surface treatment agent for substrate, comprising perfluoropolyether group-containing phosphate compound

A surface treatment agent, comprising a perfluoropolyether group-containing phosphate compound, wherein the perfluoropolyether group-containing phosphate compound is a compound represented by formula (1):

• B1—(CmF2mO)n1-A1 (1)
• wherein A1 is a monovalent organic group having at least one phosphoric acid group at its terminal, B1 is RF1—O— or D1-Q1-O—CH2— and a compound represented by the formula (1) wherein A1 is a monovalent organic group having at least one phosphoric acid group at its terminal, B1 is A2-O—, wherein a content of the compound represented by the formula (1) wherein B1 is A2-O— is from 10 to 60 parts by mass, per 100 parts by mass in total of the compound represented by the formula (1) wherein B1 is RF1—O— or D1-O1—O—CH2—,
• wherein RF1 is a C1-6 perfluoroalkyl group, D1 is CF3— or CF3—O—, O1 is a C1-20 fluoroalkylene group comprising a hydrogen atom, a C2-20 fluoroalkylene group comprising a hydrogen atom and having an etheric oxygen atom between carbon-carbon atoms, a C1-20 alkylene group, or a C2-20 alkylene group having an etheric oxygen atom between carbon-carbon atoms, and A2 is a monovalent organic group having at least one phosphoric acid group at its terminal,
• m is an integer of from 1 to 6, and
• n1 is an integer of from 1 to 200, provided that when n1 is at least 2, (CmF2mO)n1 may be one composed of at least two types of (CmF2mO) different in m.