I'm putting on my desktop computer on steel rack as usual. A while ago, when I touch the steel rack for seconds, practically I feel a tingling sensation, sort of electrical shock in my finders. After that I measured how voltage the steel rack has, the multi-tester meter indicated almost 50V by AC mode. Is this safe to touch or keep the PC on the position, or I should get away the PC somewhere electricity-safe? I'm not sure this site is suit for asking this kind of question, but I'm very worrying whether this is safe or not since this experience is my very first time. Thank you.
A comment by the OP:
In Japan almost all outlets have no third pole for grounding, only two poles they have.
Because modern SMPS wall warts (or equivalent) don't tend to use the ground wire in the socket because many times it's just not available, they have to reliably reduce emissions on the DC output wires by connecting a capacitor from output to rectified line voltage as per this answer: -
Now, the impact of adding the capacitor in the red box is that it can pass a small (safe) amount of AC current through to the secondary and, if the secondary isn't earthed via the laptop or PC then you will feel a (harmless) tingling sensation if you make contact with it.
Any device such as a computer that has a mains input with ground pole really should be connected to a mains socket that provides ground. And with a supply cord that connects the grounds. A standard ATX power supply connects mains ground to metal case. If it is not grounded, the EMI filter capacitors from Live to ground and Neutral to ground will act as a capacitive voltage divider and thus PC metal case measures half of mains AC voltage. Not hazardous itself, but connecting other mains powered devices should be made while mains cords are unplugged to prevent discharge currents from destroying devices. The slight shock can also surprise people and their reflexes can cause movements that might hit something.
The case of the computer should be grounded. If in doubt, the steel rack can be grounded as well. A stray 50V suggests that there is no ground, and you are feeling some small current leaking to the rack, perhaps via a power supply capacitor (see the answer by Andy aka).
Check your wiring - it may be in a dangerous state.