To provide a phase transfer catalyst which attains high degree of alkyl etherified substitution and to provide a method for manufacturing alkyl etherified cellulose (derivative) by using the phase transfer catalyst.

The phase transfer catalyst consists of a specific ammonium salt and difference of energy levels between a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO) of a cation molecule of the ammonium salt computed by first principle calculation using program system GAUSSIAN is 0.1 to 0.60 a.u.