This mornig the pressure was 9.8*10^-1 Pa, though it was 5.0*10^-3 Pa yesterday...
I am suspecting the residual water caused the problem.

I took a data during thus vacuum test.
The measured output voltage from ANELVA M-601GC has a relationship between the pressure which is described as

P = 10^2*V-12,

where P is the pressure and V is the output voltage.
I roughly checked the pressure using this relation and it was like 10^-6 Pa, though the displayed pressure was ~10^-3 Pa.
The output voltage was not correct for some reason,

I will check the measured voltage and do the test again without venting.